Flatten a PDF so it stops being editable

Flatten means two different things and the difference is worth a moment, because picking the wrong one either fails to solve the problem or destroys something you needed.

The first meaning is a filled form that should stop being a form. A PDF form is a set of live widgets sitting above the page: the text you typed is stored in the field, not painted on the paper, and the next person to open the file can click into it and type something else. Locking the fields writes each value into the page content and removes the widget, so the document reads exactly as it looks and nothing in it can be edited. Every page stays real text, which means it can still be searched, copied from, quoted, and read aloud by a screen reader. That is the right answer for a completed application, a signed invoice, or anything going to someone who should read it rather than continue it.

The second meaning is heavier. Some documents carry annotations, stamps, signature images, or layers that a reader can select, move, or delete because they are separate objects rather than part of the page. Rendering each page to a picture ends that: the page becomes one image and there is nothing left to grab. It also ends the useful things. Nothing can be selected or copied, the text is invisible to search and to assistive technology, and the file is usually several times larger. Choose it when the layout has to survive being opened in an unpredictable reader, and not because it sounds safer.

Neither mode removes anything the page was hiding. This is the part most guides get wrong. A black rectangle drawn over a paragraph is a rectangle, and the words underneath it are still in the file: locking the fields leaves them exactly where they were. Rendering the page destroys them, but as a side effect of keeping nothing but pixels rather than as a promise, and a promise is what redaction has to be. When the requirement is that specific words are gone, use redaction, which cuts the characters out of the content stream and then reopens the finished file to check that they are gone.

The work runs in your browser. Locking fields is quick and touches only the form. Rendering pages is real work on your own processor, roughly proportional to the page count, and pages are drawn at twice their nominal size so the result still holds up when a reader zooms in. A file with no form fields is handled honestly rather than silently: the result says there were none to lock, which is usually the sign that the picture mode is the one you wanted.