Problem opening PDF files on iPad mini

I have an iPad mini running OS 11.2.1, and am having trouble opening PDF file attachments through a SmarterMail Enterprise 14.5 email system, opened with Safari.

I have no trouble opening the files from the exact same website on a Windows 10 laptop, however when I try to open pdf attachments on iPad or iPhone, they come up as XML files? It suggests opening with iBooks, but comes up with a content not found error when I try that.

All the emails originate from the one business domain, any reason these attachments don’t work like PDF attachments on Gmail and other mail readers?

Has anyone else had this issue on Apple OS, or is it probably specific to SmarterMail Enterprise or the Safari app?

I’ve attached a picture of what happens when I try to open the files.

Thanks!