Q: Lost pdfs

Do you have copies of them in the Books section of your computer's iTunes library so that you can sync them back to your iPad ?

If they are not on your iPad, and they aren't in the Books part of your computer's iTunes library, then can get new copies of them from wherever you got them from originally ? If you have copies of them on your computer, but not in iTunes, then you should be able to add them via File > Add To Library and you can then sync them back to your iPad

What have you done with your iPad since you last had them on it e.g. have you synced to your computer's iTunes, updated the iOS version ?