Lecture slides, a reading, a form: a PDF becomes pages of a notebook you write over, and a notebook goes back out as a PDF of its pages.
Importing a PDF
Open the notebook's menu and choose Import PDF. Each page of the document becomes a page of the notebook, drawn under your ink on white paper whatever the theme, because a document's own marks are black.
A fresh notebook, one page and nothing written, takes the shape of the document: its pages replace the blank one and the notebook adopts the PDF's own page size, so a slide deck arrives as slides rather than letterboxed on A4. A notebook you have already written in keeps its shape, and the document's pages are added after the ones you have, at the notebook's own page proportions.
The hundred page cap still applies, so a long document is cut at the room the notebook has left.
Annotating
Everything on the tool bar works over an imported page: pen, highlighter, shapes, text boxes and pictures. The page image sits under the ink, so the eraser only ever touches what you drew.
The document's printed text is not read by handwriting recognition, only your ink is.
Exporting a notebook
Export as PDF in the notebook's menu writes every page, ruling, imported page and ink, at the paper's real size, so an A4 notebook prints as A4. The result lands in the share sheet on mobile and in a save dialog on desktop.