How to sign a PDF
Drop in the PDF and its pages appear in a column. Press Signature and make one: draw it with a mouse, pen or finger, type your name and pick a handwriting style, or upload a photo of your signature on white paper. Then tap the page where it goes. Drag it to move it, pull the corner handle to resize it, and use the cross to remove it. Initials, Date and Text work the same way, and you can place as many as you need on any page. Press Save signed PDF to download it.
What is changed
Only what you add. The signature goes onto the page as a picture and the date and text as real text, and every page is otherwise left exactly as it was, so its text stays selectable and nothing is redrawn. The file is saved as a copy ending in -signed.pdf.
Nothing leaves your device
The PDF is opened and signed in this page. It is never uploaded, and nothing is sent to anyone: once it is saved, you choose who gets it. Your signature is forgotten when you close the page, unless you tick Remember on this device, which keeps it in this browser only.
Questions
Is this a legally valid signature?
It is a simple electronic signature: a picture of your signature placed on the page, much like signing a printout and scanning it. Many everyday documents accept that, but it is not a certificate-based digital signature: it does not prove who signed or lock the file against later changes, and some contracts and official forms need more. If in doubt, ask whoever asked you to sign.
Can I send it to someone else to sign?
No. This is for signing a document yourself. It does not email anyone, track who has signed or keep a copy. Send the saved file however you like, and the next person can sign it here too.
Why does my typed signature look different on another computer?
The handwriting styles use script fonts your device already has, so they vary a little between systems. What you see when you place it is what is saved, because it is saved as a picture.
Can I sign a password-protected PDF?
Not if it needs a password to open or is locked against changes. Remove the protection in the program that made it first.