How do I digitally sign PDFs in 2019?

I use DocuSign, which is free web app (for single signatures). It also serves as a (hopefully) trusted third party.

From DocuSign - Wikipedia:

DocuSign, Inc. is an American company headquartered in San Francisco, California that allows organizations to manage electronic agreements. As part of the DocuSign Agreement Cloud, DocuSign offers eSignature, a way to sign electronically on different devices. DocuSign claims it has over 475,000 customers and hundreds of millions of users in more than 180 countries. Signatures processed by DocuSign are compliant with the US ESIGN Act. and the European Union's eIDAS regulation, including EU Advanced and EU Qualified Signatures.

Here's how to add a picture into an existing PDF file.

I would sign the pdf with GNU Privacy Guard (GPG). There is a list of frontends, I use GNU Privacy Assistant (GPA, apt install gpa). It works pretty well.