Pdf files | track visits

Is there any idea to track piwik in acrobat files?

For example, I have an acrobat file in my website.
After a Google search, a visitor finds a direct link to my pdf file and opens it.

It does not count as visit…

Any idea in the future to count these visits?

This is not possible directly in the PDF.
What you could do is to have a PHP script serving the PDF files, and add a call to Piwik Tracking API in the php script. http://piwik.org/docs/tracking-api/