Download and unzip the file from releases and place the nextcloud.app into the "applications" folder of your pocketbook. Once you disconnect the Pocketbook from the PC, the application should be visibile in the application launcher.
To login type the servername (e.g. https://domainname) or the WebDAV URL (e.g. htts://domainname/remote.php/dav/files/UUID) (You can look up the WebDAV URL in the files app->seetings.), Username and Password. If you have 2FA enabled, you have to set up an App specific password. (https://docs.nextcloud.com/server/latest/user_manual/en/user_2fa.html#using-client-applications-with-two-factor-authentication)
Next you will be asked where you want to save the nextcloud files. To download a file, click on it. If you want to sync a folder click it until an menu appears. In this menu select "sync". The folder sync will only sync files that are "newer" on the server side. It ignores .sdr files.