Keepmark has its own synchronization engine built into the application. With it, you can synchronize files with a remote server using the WebDav protocol, or synchronize with another local folder (and for example, encrypt the data with the Cryptomator application).
Here you can see short video tutorial about synchronization engine:
You can also sync your documents using 3rd party services - like Dropbox or GDrive.
Keepmark keeps your files and data with original format. Repository is a regular directory with list of files and subfolders, so you can synchronize all repositories using any synchronization tools. You can even access your files without Keepmark (but why sombody would like to do that when Keepmark is nearby).
This is one of my main assumptions when working on Keepmark - files and data can not be trapped in a format unavailable to other applications.