Hi Simon Brocas,
Thanks for reaching out. As described above, while saving the file you are getting the option to save as and it ask you for the location, correct?
This is a designed behavior, there is no way to default the file location to any particular folder in Adobe Reader.
You can try disabling the protected mode at startup to save a file, however, Protected View feature for PDFs is to keep your computer safe. With Protected Mode enabled, all operations required by Acrobat Reader DC to display the PDF file are run in a restricted manner inside a confined environment, the “sandbox.” For detailed info, refer to Protected View feature for PDFs (Windows), Adobe Reader
navigate to Edit>Preferences>Security(Enhanced)>uncheck 'Enable Protected Mode at startup'.
You can disable these two options in Adobe Acrobat Reader DC Preferences and see if that helps. Navigate to Edit>Preferences>General>Uncheck these two options "Show online Storage when opening files" and "Show online storage when saving files"
Let us know how it goes
Regards
Amal