Issue/Question
How do I sign a PDF using Adobe Reader or Adobe Acrobat?
Resolution
The most up-to-date Adobe Acrobat or Adobe Reader will be needed. It can be downloaded from here. Other programs, including the PDF readers that come with macOS (Preview), Windows 8+ (Reader), and any web browser with built-in PDF reader capabilities will not have full functionality of Acrobat or Reader and may not give the ability to sign a PDF.
For a step-by-step guide on signing a PDF, see this Adobe guide.