On 2018-10-08 (Mon) at 23:02 (ZZZ), Eli Zaretskii <eliz@gnu.org> wrote:
>> From: Jordan Wilson <jordan.t.wilson@gmx.com>
>> Date: Mon, 08 Oct 2018 15:44:55 +0100
>> 
>>   File error: "Opening output file", "Invalid argument", "c:/Users/Jordan/Emacs/.cache/docview-cache/ *mm*-012345" 
>> 
>> this leaves a buffer showing the file literally. If I rename that buffer
>> to something not containing an asterisk, it displays fine when running
>> "doc-view-mode". Windows apparently can't handle asterisks in
>> directory/file names.
>
> Windows doesn't allow '*' (and a few other characters) in any file
> name, not just in directory name.  Gnus should run the file name
> through convert-standard-filename to get something that is guaranteed
> to work on any filesystem.
Looking, it seems to be on doc-view's side -- if a buffer isn't
associated with a file, it uses the buffer name for the file's cache
directory. Here's a patch to the 26 and trunk branches of doc-view.el to
run the file names through `convert-standard-filename'. It fixes the
problem for me, at least.