Only one file can be uploaded for each slot unless the buyer has allowed multiple documents to be uploaded. You'll be able to tell if multiple files are allowed in the Requested Information section under the column #Files: 

For slots allowing multiple files, there is no limit to the number of files you can upload!

In some cases where only 1 PDF file is allowed for a slot, and you need to combine multiple PDFs documents into one file.

NOTE: Regardless of the number of files you can upload per Requested Information, keep in mind the limit of how large each individual file can be. For more details, take a look at our What is the Maximum File Size I can upload? page.