My host doesn't allow files larger than 8mb to be uploaded (no php override - Just Host) and Paid Downloads doesn't allow FTP uploads.
I found an easy work around.
1. Upload a small file as a placeholder for your large file here: /wp-content/uploads/paid-downloads
2. Paid download creates a "dat" file in this directory. Make a not of the file.
3. FTP upload the file you want to sell to /wp-content/uploads/paid-downloads.
4. Rename the file you're selling to the "dat" file name (remove the original "dat" file of course)
5. That's it! It works fine after that