Description is misleading, or at a minimum, easy to misinterpret. Claims to sign PDF's 'electronically', and then says 'insert any existing digital signature image from your local disk."
TRUE digital signing means the insertion of a unique encrypted certificate ID, that is applied for and issued, much like a website certification, or the digital signing of an application. Read this ADOBE article https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSig/Acrobat_DigitalSignatures_in_PDF.pdf
Unfortunately, the term digital signing is still used to refer to simply adding a scanned image, i.e. a picture, of a hand written signature. This is carry over from the days prior to true digital certificates, when a digital signature meant a scanned image, referring to the fact that the signature was added via software, versus physically signing a hard copy of a document. So do not be mislead by the developers implying that the app handles 'digital' or 'electronic' signatures. It is just image insertion, no different from stamping or placing a watermark.
In the business world and legal world, when it matters, the use of scanned image of signature is on the decline, and the preference is for digital ID signatures, because it has become too easy for anyone to stick a scanned image of anyone's signature into a doc.
This Sign PDF app, only deals with adding a scanned image, it does NOT handle true digital certificates. In fact, everything this app does, is already available through Apple's Preview app. In fact, in one sense, Sign PDF is worse, because after you add annotations (text boxes etc), and save the PDF, you can't go back and edit. Preview can save edits, and you can return later and make changes.