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
