What PDFs are people selling?
Updated
The documents people actually pay for
The PDFs people sell are mostly workbooks, checklists, templates, meal and training plans, study notes, swipe files and industry guides. The ones that earn solve one narrow problem for one kind of person, and get bought because they save time the buyer already values. General-interest ebooks sell far fewer copies.
Nearly all of them could be described in one sentence that names the buyer and the problem.
What the sellers have in common
The document is specific enough to be searched for. Somebody types the exact problem into a search box and the product is the answer.
It is also finished. A filled-in template, a tested checklist, a plan with real numbers in it — not a set of principles the reader still has to apply.
The saturated end
Broad self-improvement guides, generic business advice and beginner ebooks on crowded topics compete with an enormous amount of free writing.
They can still sell, but only to an audience that already knows the author. They do not sell to strangers.
How to find your own
Look at what people in your field keep rebuilding by hand — a spreadsheet, a checklist, a brief, a script. That thing is a product.
Search for it before you make it. If several people already sell it, that is demand, not a warning. Make the version for one specific group they are ignoring.