iPhone and iPad: Library and reader
The Library lists every PDF and scan, newest first, with All, Scans and PDFs and a search that also finds recognised text. Open adds a PDF from Files; Lens scans. In the reader, tap the page capsule to go to a page, Search to find words, Appearance for Lantern, Night or E-ink, and the modes Annotate and Fill & Sign.
Better PDF for iPhone and iPad is free.
Library
Everything you open or scan lands in the Library, newest first, under Today and Earlier. The filter switches between All, Scans and PDFs, and Search your library finds titles and recognised text.
The buttons at the bottom are Open (a PDF from Files) and Lens (scan). Swipe left on a row to Delete. The Tools row above the list has the converters (see Tools and Export). The round button at the top right opens Settings.
Read
Tap a document. The capsule at the top shows the page; tap it, type a page and press Go. Search finds words (Find in document).
Appearance has three looks, Lantern, Night and E-ink, plus Text size with Reflow text (one column at your size), and Page layout: Single, Continuous or, on iPad, Two-up. In E-ink, tap the left or right edge to turn the page.
Annotate
Choose Annotate: Highlight, Underline, Strikeout, Note, Draw (with Apple Pencil or a finger), Text, five colours, Undo and Redo. Tap an annotation to delete or move it.
Fill & Sign
Choose Fill & Sign: tap a form field to fill it, or use Text, Check, Date, Sign and Initials, dragging a box where each goes (tap for the usual size). The signature sheet has Draw, Type, Image (from Photos, Files or the camera, with Clean background and Black ink) and Saved. Certificate signing is on the Mac.
More
More has Protect with Password…, Remove Password… and Revert to Original…. Share opens Export: convert a copy, Send to Mac, Save to Files or Share….