$ file document.pdfĭocument.pdf: Java serialization data, version 5 On inspection, I noticed that it's detected as Java serialisation data, version 5. I believe that Zathura depends on detecting the file magic value (first few bytes) because it can open other formats besides PDF. I have another reader (mupdf) that does open it. I have a PDF file that my reader (Zathura) would not open.