Which PDF reader are you using? If it's the one built into your web browser (such as when you open a PDF in Firefox) then the filling of forms won't be supported. Some other PDF readers, such as the (otherwise wonderful) SumatraPDF, also don't support forms.
Firstly, ensure that you've got a PDF reader which supports forms on your computer. Much as I dislike the program (because it's cumbersome and often slow to load) Adobe Acrobat Reader is the obvious choice (because Adobe invented the PDF format and it's their software which the people at HMCTS expect you to be using):
https://get.adobe.com/uk/reader/
(NB: If you've not already got the program on your computer, so that you need to download and install it, DESELECT the 'optional offers' before doing so!)
Then RIGHT-click this link
http://formfinder.hmctsformfinder.justice.gov.uk/n316-eng.pdf
and select 'Save Link As' (or similar, depending upon your browser).
Save the file to a convenient location on your computer (such as your 'Documents' folder) and then open it with Adobe Acrobat Reader. You'll then find that you can fill in the fields on the form.
Repeat with this link:
http://formfinder.hmctsformfinder.justice.gov.uk/ex140-eng.pdf