Did you close/reopen Chrome after changing the flag? It didn't work for me until I did that. And make sure the Acrobat plugin is enabled in chrome://plugins, doesn't work with the Chrome PDF viewer.
However, once I flipped the enable-npapi flag and relaunched, both Chrome and Acrobat PDF viewers were enabled. But if you disable both then re-enable one you can only get one enabled at a time (enabling one that is disabled, disables the other):
- When only the Acrobat PDF plugin was enabled it sent me to the Acrobat print screen
- When only the Chrome PDF viewer was enabled, it gave me that featureless export to PDF box.
- When both were enabled it gave me the expected Chrome print screen