I think what you mean is to create another table for Files that has an ID field (related to your original table) and a container field for your file. You can then set up a portal to your File table that will allow you to insert the pdf files in those containers. In order to open the pdf files from those containers you would need a "export file" or "send event" type of script. Personally I have used the export file step with path set to Get)temporary file path) and the open file after export option
If you use the "store by reference" option when inserting these files, you can open them by double clicking the container field.
To expand on Mark's suggestion just a bit, think of adding a new record with a single container field each time that you want to attach an additional file to a given record. Looking up "portal" in FileMaker help and any training materials/books you have would also be a good idea as this is a basic design feature that is used a lot in FileMaker.
The "store by reference" works well if the database is single user, or if it is hosted and you control the folders that the files are stored in. If it is a shared file accessed by multiple users, there are things you can do to force the files to be stored as reference in a particular folder on the server (without the users knowing that it is happening) so that the container can then be double clicked to open by anyone else. (if Joe adds a file from his desktop stored as reference, Bill will not be able to open it since he does not have the file on his desktop). You may also want to think about whether these files will be edited by the users and how changes will be tracked if needed.