If I understand correctly, the file doesn't open when opening directly but when you upload it on Dropbox and then open it through the mobile application Adobe Reader, it opens fine.
There could be a possibility the problem is with the file's location, where exactly its saved initially. There are few specific locations which are not supported in mobile app. For example- if the file shared by the sender is actually saved on shared drive which has certain restrictions then there is a possibility that mobile application would not be able to access the file form that location.
However, when you are saving the same file to your "Dropbox" and able to access it using mobile app it did work. Because Dropbox is supported in Adobe Reader mobile app.
Could you please share the exact steps how exactly you are opening the document on mobile?