1 - Aqua Mail doesn't treat PDF's in any special way
2 - There is no hard limit on the size of attachments
3 - I have no trouble sending even larger files through my Office 365 account, on mobile data and WiFi both
Therefore, it must be something on your Exchange server.
Your company's IT people should be able to investigate from their side, why exactly the server is not keeping up with the upload.
Maybe a server side anti-virus?