How can I add an electronic signature to a PDF?

You might want to provide more details. Acrobat/Reader includes e-signature tools. You can also use commenting tools, including stamps, to add something that could be considered an e-signature. Acrobat allows you to place images and graphics as regular page contents using the page editing tools. So there are many options. Without more information about your needs and intended workflow, it's hard to recommend a particular approach.