To put files together, use Merge PDF; to take one apart, Split PDF or Extract pages. Delete pages removes the ones you choose, and Remove blank pages finds the empty sheets a scanner leaves behind. Rearrange pages lets you drag them into order, Reverse page order fixes a stack scanned back to front in one click, and Rotate PDF turns sideways pages upright.
For printing, Resize pages puts every page on A4 or Letter, Crop PDF trims away margins, and Pages per sheet fits two or four pages on each sheet. Insert blank page and Duplicate pages add the pages you are missing.
Every one of these PDF page tools is free, with no sign-up and no limit on how often you use them. Your file is opened inside your browser and never uploaded, so you can manage PDF pages free even when the document is private.