You can do a few things:
1. You should be able to see if anyone clicked on that link by taking the URL, removing the http:// and getting a smart list in your Landing Page or Smart List filter. - Use Clicked Link on Web Page.
I suspect you will get zero here because it is off site. The other problem is there is no tracking code on the link or PDF, so this will likely not work.
What would have worked is if you directed someone to your Landing Page and then asked them to download the PDF link. Then you could know precisely who did this (if they aren't anonymous).
2. Whenever you have a link, you can add URL parameters to check in GA and Marketo if anyone clicked.
If you do this the way you did, then only GA will show any data.
The better way is to force someone to visit the landing page so they can be cookied.
The URL would look like: http://pages.marketo.com/your-file.pdf?utm_content=pdf&utm_medium=linkedin&lead-source=social
This works best with a Form Fill Out.