A PDF that looks perfect on your machine can still fail on someone else's: it won't open, prints with missing characters, renders blank pages on a phone, or can't be read at all by the software a customer, court or regulator uses. "Compatibility" problems like these have a small number of concrete causes — and most of them are checkable before you hit send.
What "compatible" actually means for a PDF
A PDF is a container with strict internal rules (the ISO 32000 standard). A file is compatible with a given reader when it follows those rules for a version the reader understands, carries everything it needs inside itself, and doesn't rely on features the reader lacks. In practice, failures come from five places:
1. PDF version
Every PDF declares a version, from 1.0 (1993) through 1.7 to PDF 2.0. Almost every reader in use today handles 1.4–1.7, which is why version alone is rarely the culprit — but PDFs produced by modern tools using PDF 2.0 features can misbehave in old readers, and very old PDFs can use long-deprecated compression that new mobile viewers no longer ship.
2. Fonts that aren't embedded
The most common real-world failure. If your document uses a font that isn't embedded in the file, the reader on the other end substitutes whatever it has — which shifts layout, breaks alignment, and can render symbols or blanks where characters should be. This is also why a PDF can print differently from how it looks on screen.
3. Structural damage
A PDF's objects are indexed by a cross-reference table at the end of the file. Interrupted downloads, e-mail gateway rewrites, and buggy generators produce files whose table doesn't match reality. Forgiving readers (Adobe, Chrome) quietly rebuild the index; stricter ones — and most automated systems — refuse the file. If a PDF "works for you but not for them", suspect this.
4. Password protection and permissions
An open password blocks any reader without it. But permission restrictions (no printing, no copying) also silently break workflows: submission portals, print shops and archiving systems often reject files carrying any encryption at all.
5. Missing accessibility structure
Increasingly, "will it work everywhere" includes "will it work for everyone". A PDF without a tag structure is incompatible with screen readers and other assistive technology in exactly the way a corrupt file is incompatible with a viewer: the software cannot process it. Since the European Accessibility Act, this form of incompatibility also carries legal weight for customer-facing documents — see our guide on who the EAA covers.
How to check a PDF before sending it
- Structural and accessibility check: our free checker opens your PDF the way software does. The instant in-browser check (nothing is uploaded) catches files that fail to parse, password protection, and missing tag structure; the full audit validates the document's internal structure against the PDF/UA standard, rule by rule — files that are structurally broken fail loudly here.
- Font embedding: in Adobe Acrobat Reader, File → Properties → Fonts. Every
font should say "Embedded" or "Embedded Subset". In free tools,
pdffonts(Poppler) prints an "emb" column — every row should read "yes". - The cheap cross-check: open the file in a different reader from the one that made it — a browser and Acrobat behave differently enough that a file passing both is usually sound.
Fixing an incompatible PDF
The reliable fix is almost never repairing the PDF by hand — it's re-exporting from the source document with better settings: embed fonts (most Word/InDesign export dialogs have a checkbox), export with structure tags enabled (see our Word export guide), and avoid "print to PDF" when a real export option exists — printing discards structure that exporting preserves. If the source is gone, "Print to PDF" from a reader that still opens the file produces a clean-but-untagged rebuild — a last resort, since it destroys accessibility structure along with the damage.
General information, not legal advice.