How to populate multiple content URLs in an email based on form selections?

No, it's not the full URLs - can I insert full URLs in the values? I've never tried that.

I don't actually recommend it as it makes the setup significantly harder to maintain. Was just checking to see if you needed a separate map of names to URLs (which you do).

If the name of your field were Last Selected Documents then you'd use a Velocity (Email Script) token like this:

#set( $documentNameToURLMap = {
  "apple" : "www.example.com/apple.pdf",
  "pear" : "www.example.com/pear.xls",
  "pumpkin" : "www.example.com/pumpkin.doc",
  "orange" : "www.example.com/orange.jpg"
} )
#set( $documentNameList = $lead.LastSelectedDocuments )
## --- No need to edit below this line! ---
#if( !$documentNameList.isEmpty() )
Click for each document:<br>
#foreach( $name in $documentNameList.split(";\s*") )
<a href="https://${documentNameToURLMap[$name]}">$name</a>
#end
#end

As you can see, the first section sets up a map between each friendly name (from the form) and its corresponding URL (without the protocol).

Then it's a loop over the current values in the semicolon-delimited string (which is how Marketo always stores multi-valued fields).

Make sure to select your equivalent of Last Selected Documents from the tree on the right-hand side of Script Editor. Temporarily drag it onto the Script Editor canvas to see its Velocity name (which doesn't have spaces and isn't necessarily the same as the name you'd use in a {{lead.token}}). Then replace $lead.LastSelectedDocuments in the code (line #07) with that appropriate field name for your instance.