Screen or chat: 150. Print: 300. Thumbnail: 72.
Text and line art: PNG. Photo pages: JPG.
Converting a whole document is slow and huge; use a page range for the few pages that matter.
72dpi is screen preview, roughly 1000px wide. 150dpi covers web and document screenshots. 300dpi is the print standard. Size grows with the square of DPI, so 72 to 300 is more than ten times heavier.
Pages with type, lines and tables belong in PNG — edges stay clean. Pages that are essentially photographs are far smaller as JPG. Saving a text page as JPG leaves colour noise around every glyph, which gets worse when zoomed.
PDF content is vector; the moment you export an image it becomes fixed pixels and any enlargement is interpolation. If the goal is zooming in later, keep the PDF.
Raise DPI to 300 and switch to PNG. JPG artefacts around glyphs read as blur.
There is no fixed limit; device memory decides. Dozens are fine, hundreds should be batched.
Not directly. That needs OCR, because the original text layer is gone.
Every page becomes an independent bitmap with no shared resources. A 300dpi colour page can run several MB.