How to sign documents on SharePoint

Can someone please help me understand how our users can sign documents on SharePoint?

For example, our staff save PDF documents onto SharePoint and then require another person within the company to sign them. This document could be a document generated from a 3rd party program or from a Banking Program, or just Microsoft Word, etc.... 

When they go into SharePoint, and open the document - it opens in Adobe Cloud, (via the Adobe Toolbar we have installed) but I cannot find anywhere that allows the staff member to insert their signature and sign the document.

What am I missing?

Thanks for any help.