Command lines are handled differently on each operating system. Some characters are reserved with special meanings, even when they occur inside quoted string arguments. To avoid this problem, cpdf performs processing on string arguments as they are read.
A backslash is used to indicate that a character which would otherwise be treated specially by the command line interpreter is to be treated literally. For example, Unix-like systems attribute a special meaning to the exclamation mark, so the command line
cpdf -add-text "Hello!" in.pdf -o out.pdf
would fail. We must escape the exclamation mark with a backslash:
cpdf -add-text "Hello\!" in.pdf -o out.pdf
It follows that backslashes intended to be taken literally must themselves be escaped (i.e. written \\).