An invoice laid on a letterhead
An invoice laid on a letterhead somebody else made.
Boxes a reader fills in on screen, and your text laid over stationery somebody else already printed.
A form is a document with boxes in it: a name, a date, a box to tick, a choice from a list, a place to sign. The reader fills them in on screen and sends the file back, and what they entered comes back as data rather than as handwriting somebody has to decipher.
An overlay is the same idea the other way round: your text laid onto a sheet that already exists — headed paper a printing works produced, a contract a customer supplied, a form from an administration. The original is not redrawn and not altered. It is placed underneath, exactly as it was.
Every one of these is a real PDF this library wrote, with the source that wrote it.
An invoice laid on a letterhead somebody else made.
A supply agreement standing on three PDF files the customer sent: a headed sheet under the front page, a plain sheet under every page the schedule runs to, and a page of conditions that goes in whole.
What an unlicensed copy looks like: the evaluation watermark the library stamps on every page.
A form a reader fills in: text boxes, a check box, a drop-down, radio buttons.
A form that is also a PDF/A-3 file: every field already drawn, with a visible frame, expecting nothing from the reader's software.