The most common method of adding a digital signature to a document is by saving a background-less PNG file with your signature. Thankfully, you only need to make it once and keep it in your phone’s memory, but with the iPhone, there’s a much easier, much quicker way to add a digital signature.
To access this hidden feature, download the PDF you need to sign, then tap on the pen icon on the righthand corner. Next, use your finger to sign on the dotted line and tap Done. And just like that, you have an official document with a real digital signature.