PDF files were developed to not allow you to type into it's documents on purpose. It's meant to be that way so a document printed in it cannot be (easily) altered.
There is a way to convert it to another version which will allow you to do so, but it is fairly expensive. It's called Adobe Acrobat.
There are also ways to turn it into a jpeg document (like a photo) then paste things into/onto it, but that gets pretty complicated too... but it can be done. If you have Power Point, you can use that to past words into the spots too, but it's not simple or easy if you're just learning that stuff. It takes playing around with and practice. (I've had to do it in the past for presentations etc... but it's been a while since I retired about 4 years ago.)
Sorry I don't have better news for you... I might be able to help, if you send me the info. Lets see if anyone has any other ideas, first though. Maybe there's some other free pedigree forms out there that are easier to enter data into.