Specifying the download location for a PDF is something frequently requested, but it is not quite ready. You can download to a specific location in Drive, but not on your local computer.
In the meantime, you can set your Chrome Browser to ask each time you download a file.
This can be done by first going to the settings menu on the top right and then selecting Advanced Options at the very bottom.
From here, scroll down and tick the request box to allow you to pick the download location of your PDFs.