How PDFs work

The articles here explain behaviour that looks like a bug and is not: a rotation that reverts, a form that will not accept typing, a document that prints differently from how it looks.

How large a PDF a browser can handleHow large a PDF can a browser actually handle?There is no fixed size limit for a browser PDF tool. The constraint is device memory: tens of megabytes is routine, hundreds usually is not.
How to split a PDFWhat is the right way to split a PDF?Three different ways to split a PDF — by range, into single pages, or to fit a size limit — and how to choose between them.
Why a PDF looks different on another computerWhy does my PDF look different on someone else’s computer?A PDF that reflows, swaps fonts or loses its layout on another machine almost always has fonts that were never embedded. Here is how to check.
What makes a PDF accessibleWhat makes a PDF accessible to a screen reader?An accessible PDF needs structure, not just selectable text. What tags do, why OCR is not accessibility, and how to check a document honestly.
How to combine PDF filesWhat happens to my documents when I combine them?Merging PDFs keeps every page as it was — but bookmarks, forms and signatures behave differently. What to check, and how to get the order right.
PDF page size and orientationWhy does my PDF print at the wrong size or orientation?A4 against Letter, rotating against resizing, and why a page can look right on screen and print wrong. What each setting actually changes.
Why a PDF will not openMy PDF will not open. What is wrong with it?A PDF that refuses to open is usually truncated, password protected, or not a PDF at all. How to tell which, and what can actually be recovered.
How PDF forms workWhy can I type into some PDF forms and not others?Why some PDF forms can be typed into and others cannot, what flattening does to your answers, and the order to fill, sign and flatten in.
Why a rotated PDF goes back to how it wasI rotated a PDF, so why is it sideways again?Rotating a page in a viewer usually changes nothing in the file. The difference between turning the view and saving the rotation, and how to make it stick.
Reordering and deleting PDF pagesWhat happens when I reorder or delete pages in a PDF?Moving and removing pages changes a list of references, not the pages themselves — which is why it is instant, lossless and easy to get wrong.
What flattening a PDF actually doesWhat does it mean to flatten a PDF?Flattening merges form fields, annotations and signatures into the page itself. What that fixes, what it costs, and the order to do things in.
Getting a PDF ready to printWhat should I check before sending a PDF to be printed?Page size, margins, bleed and colour decide whether a PDF prints as intended. What to check before sending a file to a printer.
Finding what changed between two PDFsHow do I find the differences between two versions of a PDF?How PDF comparison works, why a visual diff and a text diff disagree, and what neither of them can see.
Why does my PDF look blurry?Why is my PDF blurry?Blurry on screen, blurry in print, or blurry only after compressing — three different causes. Work out which one you have.
Why an upload form rejects your PDFWhy does the application form keep rejecting my PDF?Upload forms refuse PDFs for a handful of unrelated reasons and say the same unhelpful thing about all of them. How to tell which one you have hit.
Why text copied out of a PDF comes out garbledWhy does text I copy from a PDF paste as nonsense?A PDF draws glyphs, not characters. Why copied text arrives as nonsense, without spaces or in the wrong order, and which of those can actually be fixed.
Why the page numbers in a PDF do not match the pagesWhy don’t the page numbers in my PDF match the numbers printed on the pages?A sheet can have three numbers: its position in the file, the label the document carries, and the ink printed on the page. Only the first is ever counted.
Why bookmarks and links disappear when you merge or split a PDFWhy did my bookmarks vanish after merging two PDFs?Bookmarks are not stored on the pages. They hang off the document itself — so a tool that builds a new document out of copied pages leaves them behind.

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