Python code to apply a png image Watermark to Multiple PDF files at ONCE. Steps to use this code:

1. Place all your pdf files to be watermarked in the directory 'in-pdf'.
2. Replace the watermark.png file with the png image you want to appear as the watermark.
3. Run the code.
4. All the watermarked files appear in 'out-pdf' directory once the process is completed.

Requirements: You need to install the library PyPDF2 for this code to work.