Signing on a phone
The signature box works with touch, so you can sign with your finger on a phone or tablet. Turn the phone sideways for a wider box, sign slowly, and press Use signature. If the result looks shaky, press Clear and try again; only the drawn part is kept, with the empty space trimmed away.
What kind of signature this is
The tool places an image of your handwritten signature on the page, which is what most people mean by signing a PDF and is accepted for many everyday documents such as offer letters, quotations, school forms and tenancy paperwork. It is not a certified digital signature backed by an identity provider. Some government and banking processes ask for that instead, for example signing with UAE PASS in the UAE. If you are not sure, ask whoever requested the signature which kind they accept.
Keep your signature safe
Your drawn signature is not saved on this site or in your browser; you draw it fresh each time. Because the PDF is processed on your own device, the document is not stored on any server either. Send signed documents only to people you expect, and consider adding the date next to your signature so it cannot easily be reused on another page.
More you can do with the same file
Signing uses the same editor as the PDF editor, so before you download you can also fix a typo in the text, fill in form fields, white out a line or add a stamp image. To combine the signed file with other documents, use Merge PDF afterwards.