A PDF keeps a document looking the same on every device, which also makes it awkward to change. The tools above each do one job, so the quickest way to finish is to match the job to the tool instead of trying everything.
Changing what is on a page. Use Edit PDF to add text, images or a signature. If the PDF is a form, PDF Form Filler is simpler because it lets you type straight into the fields and then print or save. Be aware that a scanned PDF is only a picture of a page: you can place new text over it, but you cannot edit the original words.
Changing which pages you have. Merge PDFs joins files in the order you set, Split PDF pulls out the pages you pick, and Rotate PDF fixes pages that were scanned sideways. Add Page Numbers is worth running last on a merged file, so the numbers run through the whole document.
Making a file easier to send. Compress PDF shrinks the size, mostly by reducing the quality of embedded images. Text stays sharp, but photos and scans can look softer, so keep your original and check the smaller copy before you send it. Email systems often reject attachments over about 20 to 25 MB, which is when compression helps most.
Protecting and opening files. Protect PDF adds a password that is needed to open the file; write the password down, because a lost one cannot be recovered. Unlock PDF removes a password you already know so the file opens without it. Repair PDF tries to recover a damaged file, but a badly corrupted file may not be fully recoverable.
Sending pages elsewhere. PDF to Images turns pages into JPG or PNG for slides or social posts, and PDF to CAD converts vector drawings to SVG for CAD programs.
A sensible order for a big job: rotate pages, merge files, remove pages you do not need, add page numbers, compress, and only then add a password. Doing it in this order avoids repeating work, because compressing or merging after protecting a file is not possible without unlocking it first.