Why a confident wrong answer looks right
Ask a sixty-page tender document what the penalty for late delivery is. If the clause is not there, most document tools answer anyway — half a percent a week, capped at five percent — because that is what tender documents usually say. The sentence is fluent, specific and about a different contract.
The mechanism is dull rather than sinister. Passages are pulled from the file and handed to a language model, which writes an answer — and a model with no passage to work from will still write something, drawing on the thousands of similar contracts behind it. Nothing marks the difference.
So the answer arrives attached to a page, and where no page supports it the reply is that the document does not say. That gives you something to check. It does not make the citation correct — a page can be cited and misread — so open it when the answer matters.
What happens to a very long PDF
A 300-page annual report does not fit in one pass. There is a ceiling on how much text can be held at once, and most tools handle it by loading what fits and stopping. Page 214 never arrives, and you are never told.
The damage is not the truncation. It is that a question about page 214 comes back with a fluent answer assembled from the first ninety pages, and it reads exactly like an answer to the question you asked. You have no way to tell the two apart from the screen.
This one trims long files too — that limit is real and no tool escapes it. What changes is that the range it read is stated, so you know the answer covers pages 1 to 90 and nothing beyond. For the rest, split the PDF and upload the section you need.
What it cannot see inside the file
It reads text. A bar chart on page 12 showing quarterly revenue is a picture to it — the axis labels may survive as text, the shape of the bars does not. Ask what happened to revenue in the third quarter and you get the surrounding prose or nothing at all.
Scanned documents fail the same way and more completely. A photographed land record, an old bank statement, a signed agreement run through an office scanner — these are images of text with no text layer underneath. Open the file and try to highlight a word; if you cannot, neither can this.
Tables usually carry through as text, though a wide one can flatten into a row of numbers with the column headings adrift. Narrow questions survive this better than broad ones — asking for the figure in a named row beats asking what the table shows. Diagrams and photographs stay out of reach.