I too found out that virtual directory was not created in IIS for my arcgisoutput directory. Soon as I made that virtual directory in IIS and made sure the permissions would allow for web access, and it worked!
One thing to note is that my virtual directory is called "arcgisoutput", is pathed to"c:\arcgisserver\arcgisoutput", and it appears as www.fishcda.com/arcgisoutput, however if you look at the actual link that is supplied by rest it looks like the links above (e.g. http://www.fishcda.com/ArcGIS/rest/services/SampleEdit/FeatureServer/3/411/attachments/2). This may lead some to believe that you need to have this crazy long path/nested virtual directories all set up (at least that is what I thought at first look). Apparently, through the magic of ArcGIS Server it translates all this and points the file back to the simple location such as www.fishcda.com\arcgisoutput.
Hope that helps someone,
There seem to be some authentication issue going on with your web server.
If I navigate to it via the REST Service Directory, I can see that it has one PDF attachment:
http://emrgisviewer.emr-inc.com/ArcGIS/rest/services/CPR/CPR_Data/FeatureServer/2/818/attachments
but when I click the PDF link:
http://emrgisviewer.emr-inc.com/ArcGIS/rest/services/CPR/CPR_Data/FeatureServer/2/818/attachments/80...
I get an "Unauthorized" error:
Server Error - The remote server returned an error: (401) Unauthorized.
Code: 500