After a 2 sec google search, it looks like Image::Magick is what you may want to use. I am in the process of looking for some documentation.
EDIT: This is the best documentation i can find so far. Hope it helps :)

Also, at PDF::API2 it specifically tells you:

# Add an external TTF font to the PDF
$font = $pdf->ttfont('/path/to/font.ttf');
[download]

So.. make a PDF, use the image, and then write whatever text on it you want on it after importing your font with the above method.
Also take a look at this link.

EDIT2: Would it not be just as easy (or easier) to just create an html page with the image as a background, and then align the user text input and format it however you want?

EDIT3: also GD looks to do this as well without needing to convert to PDF.