Which one

Half the time the answer is the other tool

People arrive wanting to merge when what they need is to take three pages out of something large, and the other way round.

Merge when several complete documents have to travel as one. Split when one document contains several things that should travel separately — a bank statement covering a year, where only one month is being asked for.

There is a third case that looks like both: you want most of a document, minus a few pages. That is neither merging nor splitting but deleting pages, and it keeps everything else exactly as it was.