[Doitsu]

It's also relatively easy to check for password-protected files with the PyPDF2 Python library:

1. Install Python 3.x and the PyPDF2 library.
2. Save the following lines as a text file with a *.py extension.
(Make sure to copy it verbatim; in Python, indentations matter. Missing/extra spaces will cause the script to fail.)

#!/usr/bin/env python
import sys, os, glob
from PyPDF2 import PdfFileReader

def main():
    current_dir = os.path.dirname(os.path.abspath(__file__))
    pdf_files = glob.glob(os.path.join(current_dir,  '**', '*.pdf*'), recursive=True)
    for pdf_file in pdf_files:
        with open(pdf_file, 'rb') as fh:
            reader = PdfFileReader(fh)
            encrypted = False
            if reader.isEncrypted: encrypted = True
        if encrypted: os.rename(pdf_file, pdf_file + '.encrypted.pdf')

if __name__ == "__main__":
   sys.exit(main())

3. Copy the *.py file to a folder with *.pdf files in it and double-click it.

If the script worked, all password-protected files should have an *.encryped.pdf extension. If it doesn't, open a command prompt/terminal window, execute the file and post the error messages.