PDF tools
These PDF jobs look similar in a menu and do different things to the file. Merge joins whole documents. Split cuts pages out. Compress shrinks pictures inside one PDF. Turning pages into JPG leaves the PDF behind.
Join or separate
Merge when several PDFs are one packet and the order you add them is the order of the result. Split when one PDF contains pages you need on their own. Neither tool pulls a paragraph out of a page. A password-protected PDF has to be unlocked before either job can open it.
Make it smaller, or make pictures
Compress when the file is one document and the pictures are what make it heavy. Text stays selectable. If the size comes from pages you can drop, split instead of compressing. PDF to JPG is for a form or a site that wants an image of each page, not a smaller PDF. A longer note on the size choice is in the compress guide.