Unbelievable. Worked for me, too. But isn't that a bug?
Actually, I was able to track the link to a Marketo hosted PDF. I wonder if this also works for an externally hosted file.
EDIT: Yep, worked as well. Worked with both trigger and batch. So what you have to do is:
1. Click link on landing page at least once
2. Use "contain" and not "is" in trigger or batch condition
3. Remove "http://" from link