http://wiki.rubyonrails.com/rails/show/HowtoGeneratePDFs contains four alternatives
HTMLDOC
which PDFs an HTML document, not likely what we want.
PdfWriter
PDF::Writer
Ruby FPDF
based on PHP version (for which you must depend for documentation)
The Cell call looks like a natural for mailing labels
see the Tables tutorial
Testing PDF Generation
We also need a means to test that we've written what we want, where we want, in the PDF document. I think I'd prefer an interface that would allow selection of a subset of the document (a rectangle of the page) and query the text contained within it.
There doesn't yet seem to be a Ruby reader for pdf (see Austin Ziegler's comment on comp.lang.ruby).
There are some possibilities to investigate in Java. I wouln't suggest using Java code to unit test Ruby code, but for acceptance or integration testing, I think it might be OK. The short list of readers seems to be:
There are also some commercial packages.