Hello Jelmer,
When you are opening your document in Revit Viewer, are you Detaching from Central? Or are you opening the Central File to view directly?
Detaching from the Central file is considered a change in the file, thus you can't print. However, opening the project directly (even though a central file) you are able to print, and since Viewer does not allow changes, you are not altering the Central in any way.
Dan Faba
Autodesk Revit Technical Support