You could set up our PDF Stamping feature to imprint the buyer's name, email address and unique Transaction ID in the upper-left corner of every page of their copy of your PDF that they download from us -- see further details and instructions here:
http://www.e-junkie.com/ej/selling-ebooks.htm
Aside from that, we have no other way to issue each buyer a unique file, nor to set up a product with a batch of files that would issue one file from that batch to each buyer; every download link we generate would issue the buyer an identical copy of whatever single file you've provided for their product. We issue each buyer their own, unique download link that expires after the max. Attempts and Hours you specify in the product's settings. This help page explains our download delivery feature in more detail:
http://www.e-junkie.com/ej/help.file-downloads.htm
If you need to create a single product that issues a collection of files or offers a choice of files to download, this help page explains some approaches you could use for that:
http://www.e-junkie.com/ej/help.package.htm
