How do I make it mandatory for a signer to sign a document before they can download it?

Not to my knowledge, but there is a workaround you could do.  The short version is you send the document to them twice in a cascade, but the first time, all they can see is a page that says they received it.  Once they’ve signed that, they’ll then get the full text of the document.

To do this: 

1. Create a template and upload your document to the template.
2. Outside of DocuSign, create a new document that has just one page and one line of text: “I acknowledge electronic receipt of this document.”
3. Upload that document to your template.  (Do not make it an another page in your initial document, it needs to be its own file.)
4. Under the recipients section, check the box to enforce a signing order.  Call the first role “Receiver” and the second role “Signer” or whatever you want to label them.
5. Go to the next screen to place fields on the document.
6. Place a signature block for the receiver on the one-line document.  Do not place any other fields for them.
7. Switch to the signer, and place all your fields as needed.  
8. In the top right-ish, click Actions and choose Advanced Options.  Check the box that says recipients must be signers to view the documents.
9. In the right but not as far up the top, click the gear symbol next to the word Documents.  Select “Set Document Visibility”
10. Now make sure that “Receiver” can not see the main agreement and that “Signer” can see the main agreement or both documents.

Once this is done, and you send the envelope, he’ll get one email inviting him to sign the envelope. When he opens it, he’ll just see the document with the one line and its block.  Once he signs it, that triggers DocuSign to send him another email asking him to sign the envelope.  When he clicks into that link, he’ll be viewing it as the “signer” so he’ll see everything.

Clear as mud, right?