Add format pdf.

Here is the code I use to generate PDFs.
I insert in the PDF, each page generated in JPEG format.
I am on a different approach than the MR !550 (closed)
Support for single and multiple pages are functional.

To test :
1 - Single page :

scanimage -d 'escl:https://192.168.1.31:443' --mode=Color --resolution=600 --format=pdf  > test.pdf

2 - Multi pages :

scanimage -d 'escl:https://192.168.1.31:443' --mode=Color --resolution=600 --format=pdf  --batch=test%d.pdf