The target directory for the mv you gave as /pdf, you should have used ./pdf. Since there was no directory (folder) named pdf in the top-level directory /, but since you were root, mv moved each of the files it moved to the file /pdf. The only recoverable file is now called /pdf, and was the last one processed by mv.
As an aside, when I sudo find, I ALWAYS use the --target-directory switch to mv, and run the find with echo first, to ensure it does what I want, like this:
# NOTE: This demonstrates a FAIL
sudo find . -type f -iname '*.pdf' -exec echo mv --target-directory=/pdf {} \;
But, I'd probably do it like this:
mkdir ./pdf
find . -type f -iname '*.pdf' -print0 | xargs -0 mv --target-directory=./pdf
mv will show an error message if the argument to --target-directory does not exist.
I use find -print0 paired with xargs -0 to deal with file names containing spaces and other silly characters.