Use terminal or console to get to the files you want to modify.
Then use touch command like this
touch -a -m -t YYYYMMDDHHMMSS *.pdf
To update timestamp of all pdf files in folder. Real life example, if you want to change last modified and create date to 1st of September 15:16:17 ( hours minutes seconds respectively ) for file todo.txt
touch -a -m -t 20170901151617 todo.txt