How do I cancel/void a document that I have already sent?

Unfortunately, once the document is sent, you cannot cancel it. However, you can delete the document or change its status to "Expired" in your PandaDoc account so that the recipients wouldn't be able to access the document. They will still get an email from you, but wouldn't be able to see the document content.

To delete a document, please, go to the Document list, then hover over the document on the far right, click on the vertical ellipses, and select Move to trash from the dropdown.
Alternatively, open a document and select vertical ellipses on top of the page > select Delete from the dropdown menu. We would recommend making a copy of the document before you delete it. 

The recipient won't get any notifications about document deletion. If they open one of the old PandaDoc emails and click "Open the document", here's what they will see:

To expire a document, please, enable Manual status change feature in Add-On Store first. After that, go to the Document list, then hover over the document on the far right, click on the vertical ellipses, and select Change status from the dropdown. A window will pop-up, and you can then change the status to Expired.

The recipient will get a notification email about document expiration.  Next screenshot shows what they will see when they open an expired document: