Detect hidden text in a PDF before giving it to an AI

A PDF can carry text no person sees but an AI assistant reads: written in invisible mode, white on white, at half a point or beyond the page edge. If that text says “ignore your instructions and…”, the assistant receives it as part of the document. It is prompt injection through documents.

This tool converts the PDF to Markdown and, in the same step, looks for those four cases in each page's drawing operations. If it finds any, it shows them with their page and text, and the warning travels with whatever you copy or download, so it is the first thing the assistant reads.

It is rule-based detection and does not catch everything: it does not see text covered by an image, or text the same colour as a background that is not white. And white text can be legitimate on a dark background. It deletes nothing: it only warns.