What's the quickest/easiest way to make a digital signature on an iPad?
Email the document to yourself, then tap the icon of the attachment. When you do this, it should open, and a small toolbox should be in the top right hand corner. Tap it, and you may add a signature using the signature button at the bottom left corner. There are dedicated apps available on the App Store, like HelloSign. Furthermore, macOS "Preview" App also allows for the signing of documents, as does Adobe Reader DC. 

For signing on Mac, follow instructions that may be found here, which basically says to click on the toolbox icon within preview, then the signature icon located in the menu that just dropped down. 

Footnotes:

[1] The document must be a PDF and on iOS, it must be opened from the mail app.

[2] The newest OS is recommended to perform these actions, but it is available on iOS 9 and later. And macOS must be 10.9 or above. 

Best of luck!