I don't know of any tutorial offhand that specifically addresses using a "master document" that you would place into a new file. It's just like placing a graphic, though.
Start by creating a .indd file (or an Illustrator file if that seems more appropriate) that will fit into your footer area and create the entire common footer content in this file, then save. Place this file, whether .ai or .indd or any other format on the document master page of the file that will become your 40 PDFs. Next time there is a change edit the placed file and update the link in the larger file.
You do need to make new PDFs every time. If all of the versions are in a single .indd file (or combined as single files in a Book (.indb) then there is no major work involved in exporting again. Pages can be extracted to sepatate files from Acrobat Pro, or you can look for a script (ask over in InDesign Scripting) that will export as single pages directly from ID.