Jinverse Ink is a free PDF signer with three decisions behind it: the file stays in your browser, you don't make an account, and the signed PDF has no watermark.
You can draw or type a signature, place it anywhere, resize it, and add name, date or initials across as many pages as you need. Touch handwriting works too, though it's worth a test on a throwaway file before an important one.
I wrote the limit on the page in plain words, and I'll repeat it here. Ink places a signature image. It isn't a digital signature with a certificate, and it makes no legal promises.
It's made by a one-person studio in Taiwan, alongside four other free browser tools.
Most online PDF signers want the file uploaded and an account made before you can sign a simple form. I wanted one that does the small, frequent job of signing something someone emailed you, entirely in the browser, with nothing stored anywhere.