I find the sync button cumbersome to use as it is hidden at the bottom of the folders list and am wondering if it is necessary to have one at all.
I'd expect that when I'm in a file and pressing the down arrow at the top to go the list of files, at this stage at least the files listed should be synced. Also ideally if the opened file is modified from another machine, this should be detected and either updated if no changed were done or the user should be asked which version to keep.
I.e., when you go to a list of file, these should be synced, and when you're in a file you should be working on the latest version and notified if it's been modified locally and remotely.
If a forced syncing is needed for some reason, then it would be better to use the pull down way of doing it as in most other iOS apps.