How do i digital signature a pdf document

If you want to add an electronic version of your written signature please see https://support.apple.com/guide/preview/fill-out-and-sign-pdf-forms-prvw35725/mac

If you want to add a digital e-signature the easiest way is probably to google “pdf digital signature” and dig into the options offered by Adobe. There should be something there, including web based tools where you can do it online rather than with an app installed locally.