Merge PDF joins several PDFs into one. Sort them by dragging or alphabetically, remove any you added by mistake, and download a single file. Pages are copied without recompression, so quality doesn't change, although bookmarks from the original files may be lost.
Split PDF goes the other way. Type ranges such as "1-3, 5, 8-10" to get one PDF per range, cut the file every N pages, or save each page as its own PDF; multiple results download as a ZIP. If you only need some pages, Extract pages is quicker: pick the pages and get one PDF with them or one PDF per page. Remove pages is the opposite: tap the thumbnails you don't want, or type them, and keep the rest.
Organize PDF is the most flexible option when the order is wrong. Load one or several PDFs, drag pages into place, rotate individual pages, delete pages (and restore them before processing) and insert an A4 blank page wherever you need it.
Scan to PDF uses the camera on your phone or computer inside the browser. Capture several pages, drag the four corners to crop with perspective correction, apply a grayscale, black and white or high-contrast filter, reorder pages and choose A4, Letter or photo size. Nothing is sent from your phone to anywhere else. To make the scan searchable, run it through OCR PDF afterwards.
Two tools prepare a PDF for paper. Pages per sheet puts 2, 4, 6, 8, 9 or 16 pages on each A4 or Letter sheet, in reading order, to print slides or long documents with less paper. PDF to booklet reorders the pages two per landscape sheet, adding blank pages up to a multiple of 4, so that printed double-sided (flipping on the short edge) and folded in half they become a booklet. In both, text stays as text; links and form fields are lost.
Merge PDF copies pages without recompressing them, and the other page tools change the order and selection of pages rather than how they look. They all accept large documents, limited only by your device's memory, and work the same on a phone or a computer. If a PDF is password-protected, open it with Unlock PDF first, since organizing tools need access to the pages.
Typical combinations: scan pages, then merge them with an existing PDF; merge, then compress before emailing; or split a large file and keep only the part you need.