I have implemented something similar in the past.
Create a new blank ASPX page (ex: webpart page) that will be used to attach the files for the newly created list item. This page should receive via URL params the ID of the item that was created (the utility I mentioned before will pass the ID along). This should just show a busy animation to the user along with a progress bar.
The code in this new page, that will attach the documents, should do the following:
check if the list item already has the PDF files attached (protects against browser refresh)
retrieve the PDF files from a document library as Base64 encoded string. If the files are small and don't change all that often, then as an alternative you can embed them in the new ASPX page as a Base64 string - this avoiding the need to download them.
attach the files to the new List item. When it is complete - redirect the user to the List view or some other page indicating "success"
Hope this helps.