Below is a suggestion on how to resolve ArtPro+ Dynamic Marks sets containing External File Marks on different locations/servers using [MasterServer] as smart name in ArtPro+ or Automation Engine.
Create a folder in ExampleJobContainer > External File Marks.
Store all your external PDF files that you would use in ArtPro+ as External File Mark in the newly created External File Marks folder.
Create a new Global SmartName, based on your requirements, i.e String Extract, Conditional SmartName etc.
When using [MasterServer] as SmartName you can send your Dynamic Mark Sets to different locations for processing, and you will no longer get an 'invalid URL' error message in the ArtPro+ Messages side-drawer.
Launch ArtPro+ and open a file from an Automation Engine job folder (to make sure the external file mark gets correctly resolved).
Add a new External File Mark via the Marks section in ArtPro+
With the external file mark selected, you can open the Mark inspector to Insert the newly created Global SmartName via the Server variables category > Global. Another example if used conditional smart name
Once all the External File Marks are created and save as Dynamic Mark sets, Zip the Folder that you had created in the ExampleJobContainer and send that to the external users or customer with the information to unzip that folder in their ExampleJobContainer on Automation Engine.