SumatraPDF CHM or DjVu Not Opening: How to Fix It

When a CHM, DjVu, or DjV document does not open in SumatraPDF, opens as a blank page, or launches in the wrong program, the cause is usually one of four things: Windows is sending the file to another app, Windows has blocked a downloaded CHM file, the source document is incomplete or damaged, or SumatraPDF is loading a persistent setting that interferes with normal operation. The steps below isolate these causes without requiring developer tools or risky system changes. Test the document after every fix. As soon as it opens and its pages or help topics display correctly, stop changing settings.

1. Confirm the Symptom With One Simple Test

Start by determining whether the problem follows one document, one format, or SumatraPDF itself. SumatraPDF supports CHM compiled help files and DjVu documents using both the .djvu and .djv filename extensions. However, format support does not guarantee that every file is complete, correctly named, or permitted to display by Windows.

1.1 Open the file from inside SumatraPDF

Launch SumatraPDF directly, select the command for opening a document, and browse to the affected file. This bypasses the Windows file association used when you double-click a file in File Explorer.

• If the file opens from inside SumatraPDF, its contents and SumatraPDF's format support are probably working. The likely problem is the Windows default app or file association.
• If SumatraPDF displays an error, an empty document, or blank content, continue with the format-specific checks below.
• If the file opens in another program before SumatraPDF appears, correct the Windows file association.

Success means the document opens in SumatraPDF and shows its expected help topics or scanned pages. If that happens, do not reset settings or reinstall the program.

1.2 Test another known-good file of the same format

Open a second CHM file if the failure involves CHM, or a second .djvu or .djv file if the failure involves DjVu. Ideally, use a document that opened successfully before or obtain a fresh copy from a trusted source.

• If the second document works, the original file is probably blocked, incomplete, malformed, or corrupt.
• If every CHM file fails but DjVu and PDF files work, focus on Windows CHM security and the CHM source files.
• If every DjVu file fails but CHM and PDF files work, verify the extension and test a newly downloaded, known-good DjVu document.
• If several supported formats fail, run the clean temporary test later in this guide.

This comparison is more useful than immediately reinstalling SumatraPDF because reinstalling cannot repair a damaged source document or change a Windows security flag attached to a CHM file.

2. Check the Format and File Association Directly

A document can appear to be a CHM or DjVu file while actually having the wrong extension, a hidden second extension, or an association with a different application. Check these details before changing SumatraPDF's configuration.

2.1 Verify the complete filename and extension

In File Explorer, enable the display of filename extensions. Confirm that the file ends in .chm, .djvu, or .djv. Watch for misleading names such as manual.chm.zip, book.djvu.html, or document.djv.part. A partial download or web page renamed with a document extension is not a valid document.

Also check the file size in Properties. A zero-byte file is empty. An unexpectedly tiny file may be an error page, download placeholder, or incomplete transfer. Download or copy the document again from its legitimate source rather than trying to force SumatraPDF to render invalid data.

Success means the file has the expected extension, a plausible nonzero size, and opens when selected within SumatraPDF. If it still fails while another file of the same format works, replace the affected copy.

2.2 Correct the Windows default app

If double-clicking opens the document in the wrong program, right-click the file, choose the option to open it with another app, select SumatraPDF, and enable the option to use that app for the file type. Repeat separately for .chm, .djvu, and .djv if you use all three extensions because Windows can maintain a different default for each one.

You can also change defaults through Windows Settings by searching for defaults by file type. Point only the relevant extensions to SumatraPDF. There is no need to reset every application default.

Success means double-clicking the file launches SumatraPDF. If opening from inside SumatraPDF already failed, an association change will not fix the document's content, so continue troubleshooting the source file.

2.3 Distinguish an association problem from a rendering problem

A file association controls which application receives the file. It does not control how pages are decoded after the file reaches SumatraPDF. If the SumatraPDF window opens but shows blank content, the association has already done its job. Focus on CHM blocking, document integrity, or a persistent SumatraPDF setting instead.

3. Apply the Correct CHM or DjVu Fix

CHM and DjVu are both supported non-PDF formats, but they fail for different reasons. CHM files are compiled HTML Help archives and are subject to Windows security behavior. DjVu files contain encoded document pages and commonly fail when the source is truncated, malformed, or mislabeled.

3.1 Unblock a trusted downloaded CHM file

Windows may mark files downloaded from the internet as originating from another computer. For a CHM file, that security information can prevent embedded help pages from displaying, even when the table of contents or document shell appears. This often produces blank topics or incomplete content rather than a simple file-not-found message.

1. Close the CHM file and SumatraPDF.
2. Right-click the CHM file in File Explorer and select Properties.
3. On the General tab, look near the bottom for an Unblock checkbox or button.
4. Use Unblock only if you trust the file and its source, then select Apply and OK.
5. Open the file again in SumatraPDF.

Success means the previously blank help topics now display. If the Properties dialog has no Unblock option, the file may not carry that internet-origin marker, or it may already be unblocked. Do not weaken Windows security settings globally to compensate.

3.2 Copy a CHM file to a local folder

Windows HTML Help security restrictions can affect CHM content opened from network shares, remote locations, synchronized placeholders, or other locations treated as less trusted. Copy the trusted CHM file to a simple local folder, such as a folder under Documents, and open that local copy.

Make sure the copy is fully available offline. If a cloud service shows a placeholder icon, request that the file be downloaded locally before testing it. Avoid opening the document directly inside a ZIP archive because temporary extraction can complicate access to the file.

Success means the local copy shows its help pages while the network or placeholder copy did not. In that case, keep a trusted local copy rather than applying broad registry changes or reducing security restrictions.

3.3 Identify a corrupt or incomplete CHM file

A CHM file can contain a damaged archive, missing internal topics, or broken links created by the help-file publisher. If one CHM fails in both SumatraPDF and the Windows HTML Help viewer while other CHM files work, the source file is the likely cause.

Download it again from the publisher, compare its file size with the source if that information is available, or ask the provider for a replacement. Unblocking cannot reconstruct missing data. Similarly, reinstalling SumatraPDF cannot repair bad internal links authored into the help file.

3.4 Diagnose an incomplete or malformed DjVu file

For .djvu and .djv failures, first obtain a fresh copy and wait for the download or synchronization to finish. A truncated DjVu file may fail immediately, show only some pages, or produce blank or damaged pages. Renaming an unrelated file to .djvu does not convert it into DjVu.

Try the same document in another reputable DjVu-capable viewer solely as a diagnostic step. If multiple viewers reject it, replace the file. If another viewer opens it and SumatraPDF does not, proceed to the clean temporary SumatraPDF test.

Success means a fresh copy displays all expected pages, page thumbnails, and navigation. Once the replacement works, delete the incomplete copy to avoid reopening it later.

3.5 Know which external components are usually irrelevant

Standard CHM and DjVu reading in SumatraPDF should not require printer drivers, media codecs, or Ghostscript. Ghostscript is associated with PostScript processing, not ordinary .chm, .djvu, or .djv viewing. Installing random codec packs or changing a printer driver is therefore not an appropriate first response to a CHM or DjVu opening failure.

Printer settings matter only if the document opens correctly but printing fails. Keep viewing and printing symptoms separate so that a print-driver problem is not mistaken for a document-decoding problem.

4. Inspect or Reset SumatraPDF-settings.txt Safely

SumatraPDF stores preferences in a text settings file. A persistent setting problem is plausible when multiple known-good documents fail, behavior returns after every launch, or one SumatraPDF installation behaves differently from a clean copy.

4.1 Find the active settings file

An installed copy commonly keeps user data in the SumatraPDF folder under the current user's local application data location. A portable copy is designed to keep its files with the portable program when that location is writable. The important point is to identify the settings file used by the exact executable you are testing.

Search for SumatraPDF-settings.txt in the relevant SumatraPDF data folder or beside the portable executable. If several copies exist, note their locations and modification times. Do not assume that editing one file changes every installed or portable copy.

4.2 Back up before making changes

1. Close every SumatraPDF window.
2. Copy SumatraPDF-settings.txt to a backup name such as SumatraPDF-settings-backup.txt.
3. Rename the active settings file rather than deleting it.
4. Start SumatraPDF and let it create or use clean defaults.
5. Open a known-good CHM or DjVu test file.

Closing SumatraPDF first matters because the application may write its current preferences when it exits. Editing or renaming the file while the program is running can cause your change to be overwritten.

Success means the test document works with clean defaults. Keep the newly generated settings and reapply only the preferences you actually need. If nothing changes, close SumatraPDF and restore the backup if you want your previous preferences back.

4.3 Do not copy the entire old configuration into a clean test

If resetting settings fixes the issue, copying every old setting back at once can reintroduce the problem. Restore ordinary preferences gradually. Test the affected CHM or DjVu file after each meaningful group of changes.

If the reset does not help, the settings file was probably not responsible. Stop modifying it and move to a clean executable test or replace the source document.

5. Run a Clean Temporary Test Before Reinstalling

A clean portable test separates the program from the installed copy, its shortcuts, and its existing settings. It is usually more informative than immediately uninstalling and reinstalling, especially because reinstallers may leave user preferences untouched.

5.1 Create an isolated test environment

1. Download an official current SumatraPDF package from the project's website.
2. Place a portable copy in a new writable local folder.
3. Do not copy your existing settings file into that folder.
4. Launch the executable directly from the new folder.
5. Open one known-good CHM and one known-good .djvu or .djv file, if available.

Use ordinary local paths for the test. Avoid launching the portable copy from an archive, network share, read-only disc, or protected system directory. These locations can prevent normal settings persistence and introduce unrelated access problems.

5.2 Interpret the result

• If known-good files work in the clean copy, the original executable, shortcut, or settings environment is the likely problem.
• If only the original document fails, replace or re-download that document.
• If CHM works only after unblocking or local copying, Windows security or file location caused the symptom.
• If double-clicking still opens another app but direct opening works, repair the file association.
• If all known-good test documents fail in both clean and installed copies, verify that they are genuinely complete files and obtain a new official SumatraPDF package.

Reinstall only after this test indicates that the installed environment is at fault. A clean test that works is also a valid temporary solution while you repair shortcuts or defaults.

6. Quick Fix Checklist

• Open the file from inside SumatraPDF to bypass Windows file associations.
• Test another known-good file with the same .chm, .djvu, or .djv extension.
• Enable filename extensions and check for .zip, .html, .part, or other hidden suffixes.
• Confirm that the file has a plausible nonzero size and is fully downloaded.
• For a trusted downloaded CHM, check Properties for the Unblock option.
• Copy a trusted CHM from a network, archive, or cloud placeholder to a local folder.
• Set SumatraPDF as the default app separately for each affected extension.
• Back up and rename SumatraPDF-settings.txt while SumatraPDF is closed.
• Test an official portable copy in a fresh writable folder.
• Avoid unrelated codec, Ghostscript, printer-driver, or global security changes.

Stop as soon as the document opens, its navigation works, and the expected content appears. Continuing to reset defaults after success makes it harder to identify what fixed the problem and may remove useful preferences.

7. Frequently Asked Questions

7.1 Does SumatraPDF support CHM, DjVu, and DjV files?

Yes. CHM is a supported compiled help format, while .djvu and .djv are filename extensions used for DjVu documents. These are non-PDF formats, but SumatraPDF can read them alongside PDFs, eBooks, comics, XPS documents, and supported image formats.

7.2 Why does a CHM file open but show blank content?

A downloaded CHM may be blocked by Windows, or it may be opened from a location affected by HTML Help security restrictions. Check the file's Properties for an Unblock option and test a trusted local copy. If those steps do not help and other CHM files work, the help archive or its internal links may be corrupt.

7.3 Why does double-clicking a DjVu file open the wrong app?

Windows has associated .djvu or .djv with another application. Open the file from inside SumatraPDF to confirm it is readable, then change the default app for the affected extension. Configure .djvu and .djv separately if both are used.

7.4 Will reinstalling SumatraPDF repair a damaged document?

No. Reinstallation cannot recover missing bytes, rebuild a malformed CHM archive, or complete a truncated DjVu download. If another known-good file opens, replace the failing source document instead of reinstalling the reader.

7.5 Do I need Ghostscript or a codec pack for DjVu?

Not for normal DjVu viewing in SumatraPDF. Installing unrelated codec packs or Ghostscript is not a targeted fix for .djvu or .djv opening failures. Test a known-good file and a clean SumatraPDF copy first.

7.6 When should I reset SumatraPDF-settings.txt?

Reset it only after multiple known-good files fail or the behavior appears tied to one SumatraPDF environment. Back up the file, close SumatraPDF, rename the active settings file, and test clean defaults. If the reset has no effect, restore your preferences and investigate the source file, Windows blocking, or file association instead.