A PDF is a fixed page. Everything in it, text, rules, a table's alignment, a form field's box, is placed at a coordinate and stays there. That is the whole reason to reach for the format and the whole reason it is awkward: a PDF is right when the layout is part of the message (a filled form, a signed-looking statement, a tear sheet that must print identically everywhere) and wrong when the user will want to edit the content later.
Two jobs live here and they barely overlap. Reading a PDF the user hands you: find out what it is, pull the text and tables, look at the pages that matter. Producing one: fill an existing form, assemble pages, or draw a new document. Both end the same way, by rendering to PNG and looking, because nothing else in this toolchain can see the page.
User preferences override these defaults. A template the user supplies, a house font, a filename convention, a specific page size: those outrank every rule here. The rules below are for when nothing has been specified.