Key facts
- MIME type application/pdf; files start with %PDF-1.x or %PDF-2.0 and normally end with %%EOF.
- An open ISO standard (ISO 32000) originally created by Adobe; internally a set of objects indexed by a cross-reference table.
- Can embed fonts, images, vector graphics, form fields, digital signatures, attachments and JavaScript.
- Sizes range from a few KB for text to hundreds of MB for scanned or image-heavy documents.
- Variants include PDF/A for archiving and PDF/UA for accessibility.
How to open a .pdf file
Windows
Double-click to open in Microsoft Edge, or install Adobe Acrobat Reader or SumatraPDF. Chrome and Firefox also open PDFs.
macOS
Opens in Preview, which can also annotate, sign, merge pages and fill most forms. Acrobat Reader handles complex XFA forms.
Linux
Use Evince (GNOME Document Viewer), Okular or a browser. Extract text with pdftotext file.pdf out.txt and inspect it with pdfinfo file.pdf (poppler-utils).
Common problems and fixes
- 'The file is damaged and could not be repaired'
- The download or email attachment is usually truncated or the cross-reference table is broken. Re-download it, then try rewriting it with qpdf damaged.pdf repaired.pdf or gs -o repaired.pdf -sDEVICE=pdfwrite damaged.pdf.
- PDF is too large to email or upload
- Large PDFs usually contain high-resolution images. Recompress them with gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -o small.pdf big.pdf or Preview's Reduce File Size filter, and check text stays readable.
- Cannot select or search the text
- The PDF is a scanned image without a text layer. Run OCR (for example ocrmypdf scan.pdf searchable.pdf) to add selectable text.
- Asks for a password or blocks printing and copying
- The PDF is encrypted with an open password or permission restrictions. Ask the sender for the password or an unrestricted copy; removing protection you are not authorized to remove may breach terms or law.
- Form fields are blank or show 'Please wait...'
- It is an XFA form that only Adobe Acrobat or Reader can render. Open it in Acrobat Reader instead of a browser or Preview.
Often converted to or from: DOCX (Word), JPG/PNG, TXT, HTML, PDF/A