Unknown file encoding when viewing pdf files online

Please tell me, seafile server 8.0.4 is installed on Ubuntu 18.04. When I try to view a pdf document online, I get an error “Unknown file encoding”. What am I doing wrong? Thanks for any answers.

Can you view the PDF document locally after downloading from Seahub?

Specifically, proceed as follows:
a.) Log on to the Seafile web interface
b.) Download the PDF file
c.) Open locally with Adobe, Foxit, …
d.) Report

You can also check Seafile’s log files for any hint that may be the problem.

Yes, I can view the PDF document locally after downloading from Seahub with Foxi Reader.

And the log files?

Log files: controller.log, seafile.log, seahub.log - no errors or warnings.

Selected ‘Open with client’ for pdf file in seafile library and it worked, thanks.