The document viewer widget doesn't load external file documents. To use it, you would have to place it in a dataview with the data source set to an entity that inherits from the filedocument entity.
Are you able to retrieve the file through a web service? One option wouldbe to use a dataview that uses a microflow as a data source that retrieves the pdf through a webservice and then use the file document viewer widget to display the pdf. The rest service activity has an option to store the response in a file document.
You could add a REST service. Add a resource and an operation. In the operation use a microflow that returns the file document.