Can't open the PDF attachment in GroupShare (2019)

Hi, 

I'm new to GS projects and accepted a project.

I can open the file to translate. No problems here, but I cannot open the PDF attachment (reference file) in the project, although it shows as available in the list with files. As soon as I try to open the PDF, the message appears that the PDF cannot be found. Has this to do with any settings? 

And can I add my own TM to a GroupShare project and use it safely as additional TM?

Thank you.

Kind regards, Pauline