You can't import the PDF's and have them function as editable forms in FileMaker.
Your best option is to recreate them on your layout item by item with the layout tools. For each "blank" on your PDF form where a person would enter data, you'll need to define a field in the database in order for users to enter data into that spot on the form.
Some users have been able to insert a PDF onto their layout so that they can put it in the back ground and then add the needed FileMaker Elements on top of it. I think they either did this with insert picture on a Mac system or converted the PDF to a graphic file format such as .jpg to do that.
At the very least, you can open the PDF file and then copy and paste the text from the PDF onto your FileMaker layout.