Does M-Files support pdf portfolio?

The classic M-Files Desktop leverages Windows previewing capabilities, so if you can find a compatible third-party application that can provide preview for these PDF portfolios in File Explorer in Windows then the preview should also work in M-Files. I'm not familiar with these PDF portfolios so I cannot tell whether such a preview application exists or whether you are expected to open them in Adobe Reader.

Note that an application that can open a certain file type cannot necessarily preview it. Previewing is a separate capability.