Which PDF tool should you use?
Start with what you want the final file to look like. To join documents, use merge. To cut one document into parts, use split. To keep a few pages, use extract, and to throw a few away, use delete. If the pages are in the wrong order or sideways, organize and rotate fix that.
Turning pictures into a document is a job for JPG to PDF, and the opposite direction is PDF to JPG. When the file is too large for an email or a form, use compress. For finishing touches on a document you plan to share, add page numbers or a text watermark.
A typical order of work
Most real tasks use two or three tools in sequence. A good order is to clean first (delete, rotate), then arrange (organize, merge), then finish (page numbers, watermark) and compress last. Compressing at the end means you do not recompress the file after every edit.
For a job application, for instance, you might photograph your certificates, build a PDF from the photos, rotate any sideways pages, merge it with your resume, number the pages and compress it to fit the portal limit.
Why these tools run in your browser
PDF files often hold sensitive things: contracts, bank statements, ID scans. Uploading them to an unknown server is a risk many people would rather avoid. Here the work is done by code running on your computer or phone, so the file does not travel anywhere. You can compare that with online services that must receive your document to process it.
The trade-off is that your device does the work. Big files may take longer on an older phone, and the limit is 100 MB per PDF.
What these tools cannot do
Password-protected or encrypted PDFs are not supported, so remove the protection before you start. There is no PDF to Word conversion, and no OCR to turn scanned pages into editable text. Compress has two modes: normal keeps text selectable with modest savings, and strong turns pages into images for a much smaller file.
If a tool is not listed, it does not exist here yet. Everything that is listed is free, with no daily limits and no watermark added to your files.