A PDF accessibility checker reads the file's tag tree and reports what is missing: untagged content, images with no alternative text, tables without header cells, no document title, no language. It is a genuinely useful gate before publishing, and it is not a verdict on whether the document reads well. This page is what the checkers test, how to check one online without installing anything, and the two manual passes that decide whether a document actually works for the person it was made accessible for.
What the checker decides, and what it only flags
It decides the machine-checkable facts: tagged or not, title set or not, language set or not, alt attribute present or absent, table headers marked or not. It flags for human review the things it cannot judge, chiefly whether the reading order is right and whether the alt text is a description or the word image. A file that passes with fifteen items flagged for review has not passed.
How to check pdf accessibility online
The free online checkers upload the file, read its structure and return the same list. Use them for a document you have just exported, before it goes on the site, and use the authoring tool's own checker while you still have the source open. The rule worth keeping is that the check happens before publication, because remediating a live document costs several times what exporting it properly would have.
The two manual passes that matter
Read the document with the tag tree open and confirm the order matches what a sighted reader follows, then listen to the first two pages with a screen reader. Those two passes find the failures no checker reports: a column read out of order, a chart whose alt text is the word chart, a table whose headers are visual rather than marked up.
Questions people ask about pdf accessibility checker
Is an online checker safe for confidential documents?
Treat anything you upload as published. For confidential files use the authoring tool's local checker instead.
Does a passing checker mean the PDF is ADA compliant?
No. It means the structure is present. Whether the document is usable is decided by the reading order and the alt text, which a person has to judge.
What about forms?
A PDF form needs every field labelled and reachable by keyboard in a sensible order. If it is a form people must complete, an HTML form is nearly always the better answer.