I receive encoded PDF files regularly. The encoding works like this:

• the PDFs can be displayed correctly in Acrobat Reader
• select all and copy the test via Acrobat Reader
• and paste in a text editor
• will show that the content are encoded

so, examples are:

13579 -> 3579;
hello -> jgnnq

it's basically an offset (maybe swap) of ASCII characters.

The question is how can I find the offset automatically when I have access to only a few samples. I cannot be sure whether the encoding offset is changed. All I know is some text will usually (if not always) show up, e.g. "Name:", "Summary:", "Total:", inside the PDF.

Thank you!