At the moment client's content editors can't add PDFs to buttons or replace the PDF file. The option does not appear on the Editor at all, and on Build Mode it does but it does not let you select the file.

At most, from Build Mode (using an Admin account) they can access the file's url within Media panel and copy the url to paste as a link on the Button, but this is only one account on their organisation, and they have a big team of Editors invited for content management, that can only access he Editor.

This thread on the Forum shows examples:
https://discourse.webflow.com/t/as-content-editor-why-cant-i-replace-attachment-on-a-button-that-opens-a-pdf-file/271197