In general terms, a PDF document which you find online is probably available on paper.

Decide what kind of item it is, eg a report, policy document, guidelines etc. (which are referenced like books), and reference using the appropriate template. If you are able to find the information you need to do this then there is no need to give an electronic reference.

If it proves impossible to find the information you need to give a full reference, then reference the PDF in the same way as you would reference a web page.