Ok so I've encountered a particular issue regarding PSN+ Cloud storage and the using of another players saves to finish their trophies. I've had this happened over 4 times now. I am not aware if anyone knows this or not so I felt like posting this bit of information:
1. If you download their save (assuming you are both from the same region), it is more than likely that you will need to "activate" one of their available slots for the save to even work. This will account for 75% of my encounters lately. Only 1 time has this not been tried, so I can't confirm that it is a perfect method for getting some of the saves to work.
2. When uploading to someone's cloud storage, you may encounted a little pain in the a*s error (I think it is like 802160009 or something similar). To correct this error, you must load up and play the game while logged onto the person's psn account and create a autosave (for some games such as pacman) or even another save file. This will ALWAYS correct that error.
Again, not sure if anyone was aware of it or not, so I wanted to post that bit of info. Since the auto-upload feature was introduced, there have been so many errors getting saves downloaded or uploaded properly. Just passing on some hard earned experience