This is akin to saying, “I’ve done something wrong; how do I fix it?” You must provide some details.
What are the steps you have taken?
What does happen (error messgae, for instance)?
What application creates an FDF?
How have you created a pdf file form this fdf? (Apparently you have [u]not</u> created a pdf file, because, by definition, anything that is a file [u]is</u> saved. “Saving” turns data into a file. If it is a file, it is saved...