What does not usually synchronize among devices using the same E-Reader app?

Account information

Bookmarks

Downloads

Purchases

The answer is Downloads.

Downloads must be saved in local storage such as memory and hard drive. If you are using two or more devices, then these items must be downloaded in your local memory and it will take time, depending on the size of the document you are reading.

Account information, bookmarks and purchases can be saved and synchronized as these are small information and sizes.