Search for a PDF tool and almost every result works the same way: your file travels to a server, the server does the work, and a sentence promises deletion within the hour. For a public brochure that is fine. For a signed contract, a payslip, a medical letter or an invoice with bank details, it means a company you know nothing about holds a copy.
These tools run the whole operation in your browser with a JavaScript PDF library. There is no size limit beyond your own memory, no queue, and nothing to delete afterwards. You can verify it: open the network tab of your developer tools and use one.
Questions
Is there a file size limit?
Only your device memory. There is no server, so no upload limit and no queue. Very large scans may be slow on an older phone.
Are password-protected PDFs supported?
Only if they are not truly encrypted. A PDF with an owner password can usually still be read; one that needs a password to open cannot.
Does quality suffer?
No. Pages are copied rather than re-rendered, so there is no re-compression and no loss.