You should download the latest Adobe Acrobat Reader. Click on the form you wish to download / fill in. The file will either download or open in a browser window. Follow these instructions:
If the form has downloaded, open it in Adobe Acrobat Reader.
If the file opened in a browser window, hover over the document near the bottom of the window unit you see this:
Selecting the 3rd icon should open it in Adobe Acrobat. The 4th icon will download the file to your computer.
Now the file is open in Adobe Acrobat, choose Fill & Sign to complete the form. Choose Save As.. on your computer and name the file. This will leave the original intact if you need to use it again.
Alternatively, Print the form off and complete by hand - neatly, post it or scan and send via email.