It sounds like you have a PocketBook Touch 622. If you go to the PocketBook website at
https://pocketbook.ch/en-ch/support?model=135 you can download the user manual and the last firmware version. The user manual contains information about doing a firmware update. If the hardware really is the same as the official PocketBook Touch, then it is probably best to update the firmware and get the applications for reading all formats reinstalled. However, if the reason you can't use the existing applications is that the memory is starting to fail and corrupting the contents, then updating the firmware may leave you with an even more broken system. Maybe you can install something that gives you access to a shell on the device and explore it further before deciding on whether to attempt to update it or leave it as is. Personally, I use utelnetd for getting shell access over the internet from my desktop, and you can search for that in this forum if you want to install it. Third party software is usually just copied to the /applications/ folder when you mount the filesystem via USB.