The normal way is to install (La)TeX (probably already on the linux box) and run pdflatex to get the pdfs. You can also use Apache FOP, if you prefer xslt and xsl-fo.
If the number of invoices to create is low you might want to use open-office (directly or as a toolkit).
If you want high-precision positioning and low-level access, a low-level pdf library (I don't know if iTextSharp works with mono) might be what you want.
I would try out LaTeX first, because it allows you to get results with the least effort.
