Which split do you actually need?
By range when you know the structure: pages 1–3 are the letter, 4–12 the report. Every page when each page is its own document, such as a batch of scanned receipts.
By size when the reason is an upload or email limit: the parts are filled up to just under the limit, so you get the fewest files rather than an arbitrary number. By bookmarks when the document has chapters: each bookmarked section becomes a file named after it.
What happens to bookmarks and links in the parts?
Each part keeps the bookmarks that point inside it. Links pointing to a page that ended up in a different part cannot follow anything, so they are left as inert text rather than broken jumps.
Page numbering printed on the pages does not change — page 40 still says 40 in the part that holds it, which is usually what you want for a court bundle or a report extract.
Do the parts add up to the size of the original?
Roughly, and often slightly more in total. Each part carries its own copy of the fonts and any shared resources, so ten parts of a 1 MB file can total 1.4 MB.
This matters when splitting to beat a limit: check the part sizes rather than assuming a 30 MB file becomes three 10 MB ones.