Most PDF sites want an account to merge two pages, then keep the file around.
SoHappyPDF is one tool per job: merge, split, compress, convert, protect. No account for basic use. Files belong to your session and expire after about two hours — not a document library.
Free is 5 ops/day and 25 MB. Pro is $5/month (OCR and higher limits). Same pipeline is on a public API.
