Knowledge base

Reference articles on how PDFs work, written to answer one question each. Every article states its answer first, then explains the detail and the limits.

PDF basics 5

What a PDF file actually isWhat is a PDF, underneath?A PDF describes finished pages, not editable text. What is inside the file, why it looks the same everywhere, and what that costs you.
Is this PDF scanned or is it text?How can I tell whether a PDF is scanned?A scanned PDF is a photograph of a page. How to tell in two seconds, why it changes what every tool can do, and what OCR fixes.
PDF or Word: which should you send?Should I send this as a PDF or as a Word document?Send a PDF when the layout must survive the trip, and a Word file when someone has to change it. What each format guarantees, and what it costs.
How to edit a PDFHow do I edit a PDF?"Edit a PDF" means five different jobs, and they need different tools. Work out which one you have first.
What PDF/A is, and why an archive rejects an ordinary PDFWhat is PDF/A, and why was my PDF rejected?PDF/A is a PDF that must carry everything it needs inside itself. What the standard forbids, and how to tell whether a file really is one.

Converting and extracting 9

File size and compression 6

Privacy and security 10

How it works 18

Original research

PDF compression benchmark How much smaller does a PDF actually get, and what does it cost? Six document types, measured against the shipped compression engine.

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