Media items of type PDF that link to an actual .pdf file on a server are automatically cached in the native apps (iOS and Android).
If you need to update a PDF file that your attendees may have already downloaded, you can simply append a ?<versionnumber> to the end of your media item URL.
Your first version you included in the app had the following link:
Imagine you have updated the file on the server. The file name on the server remains exactly the same. How would the app know that there is a new version?
You would update the URL by appending ?1 (either manually in the CMS or via your data import):
If you update the document again, change the ?1 into ?2 in the CMS:
This minor version indicator will trigger a new download of the file, even if the user has already downloaded and a cached the original version.