By default PPT 2010 opens files from the internet in "Protected View" which is basically Read Only. If you open a pptm there will be an option to allow edits, if you open in a show (ie a ppsm) this option never appears and edits are blocked
Your code will be trying to edit the show in some way and won't be allowed to do it!
If the user right clicks on the downloiaded file > Properties there may be an option to Unblock
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