The type of on-line fillable forms that the VA uses are .XFT documents which are more than likely created by Adobe LiveCycle (unless the docs are really old and were created by software called FormFlow, a product that Adobe purchased and which evolved into LiveCycle). You can learn more about how to create these types of documents here:
Although VA and several other government agencies use these types of forms, the link you provided only contained downloadable PDF forms, so am not sure what you found in the five documents on your link that could be filled out online.
If you go to this link:
VA explains that:
IMPORTANT: You must have the latest version of Adobe Reader on your computer to fully utilize all the forms on this site. You can download software at: Free Adobe Reader Download. (Note: By clicking on Free Adobe Reader Download link, you will leave the Department of Veterans Affairs Website. VA does not endorse and is not responsible for the content of the linked website)
Forms on this site are available in several formats. Fillable pdf can be filled on-line, printed, saved and edited. XFT forms can be filled on-line, printed, and saved to your pc but cannot be edited later (TURN OFF YOUR POP-UP BLOCKER TO USE XFT). Some PDF are printable blank forms.