sds@st-and.ac.uk

I had a similar problem, and I found a solution to this problem on the thread at
https://wordpress.org/support/topic/weird-chars-instead-of-download-pdf-file/

basically add

\?wpdmdl.*

to the list of url strings that force a page not to be cached in Super Cache settings.

There is a how-to about making these kind of exceptions at…

WP Super Cache plugin – How to exclude Shop Pages