PDFs are specially formatted files which are typically used for official documents or forms. One easy way to fill out a PDF form is to simply print it then write on it with a pen or pencil, however you can also fill out and sign most PDFs electronically.
Here's some tips on how to electronically fill out and sign an PDF.
First, make sure you are using the free Adobe PDF Reader software which can be downloaded at the Adobe website. Go directly to the download site. Make sure to uncheck the boxes in the middle to avoid downloading and installing the advertised additional software, that's partially how PDF Reader stays free.
NOTE: While most web browsers can view PDFs they won't necessarily allow you to electronically fill and sign the PDF. Make sure you are using Adobe PDF Reader.
For complete steps on how to fill and sign a PDF, visit the Adobe website.