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Failed to load libsumatrapdf.dll
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When installed, SumatraPDF consists of 2 files:
  • SumatraPDF.exe
  • libsumatrapdf.dll
Name change: through version 3.6 this library was named libmupdf.dll. From version 3.7 it is named libsumatrapdf.dll. The role is the same (MuPDF and the other document codecs used by the app). Older docs, antivirus logs, and this page’s URL still use the historical libmupdf name.
You’re reading this page because loading of libsumatrapdf.dll failed and we can’t proceed.
How can that happen?
There are many possible reasons. The error code and message is what Windows reports.

Antivirus / security software (most common for portable / pre-release) #

Portable and single-file builds extract libsumatrapdf.dll next to the executable or under %LocalAppData%\SumatraPDF-data\ the first time they run.
Some antivirus products:
  • block LoadLibrary on a just-written DLL
  • quarantine or delete the DLL immediately after it is extracted
  • return unusual error codes (for example 87 / “Invalid parameter”, 5 / access denied, 32 / sharing violation)
This is especially common with real-time protection and “behavior” or “ransomware” shields.
What to try:
  1. Add exclusions for:
    • the folder that contains SumatraPDF.exe (portable install)
    • %LocalAppData%\SumatraPDF-data\ (extracted build-specific files)
  2. Temporarily disable real-time scanning, start SumatraPDF once so libsumatrapdf.dll can load, then re-enable scanning (keep the exclusions).
  3. Check the antivirus quarantine for libsumatrapdf.dll (or, on older installs, libmupdf.dll) and restore it if found.
  4. Prefer the official installer from sumatrapdfreader.org if portable keeps failing.
  5. Re-download SumatraPDF (ensure the download is complete and not modified).
If the dialog shows a Windows error code, note it when asking for help.

Other causes #

  • Incomplete copy of a multi-file install (libsumatrapdf.dll missing next to the .exe)
  • Wrong / leftover libsumatrapdf.dll from another version (size mismatch)
  • Corrupted download or disk errors
  • Security policies blocking unsigned or newly written modules
How to investigate a specific error code:
  • Search: LoadLibrary <error-code>
  • Ask an AI: when calling LoadLibrary() Win32 API I got error <code>. What does it mean?
  • Ask in SumatraPDF discussions
See also Corrupted installation and Installation.
SumatraPDF documentation
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Failed to load libsumatrapdf.dll
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