pdfmom prints to stdout, so output must be redirected to a destination file. The size of the final PDF can be reduced by piping the output through ps2pdf.
If called with the -Tpdf option (which is the default), pdfmom processes files using groff's native PDF driver, gropdf. If -Tps is given, processing is passed over to pdfroff, which uses groff's PostScript driver. In either case, multiple runs of the source file are peformed in order to satisfy any forward references in the document.
pdfmom accepts all the same options as groff. If -Tps is given, the options associated with pdfroff are accepted as well. Please note that when pdfmom calls pdfroff, the
options are implied and should not be given on the command line. Equally, it is not necessary to supply the -mom or -m~mom options when -Tps is absent.
PDF integration with the mom macros is discussed in full in the PDF manual, Producing PDFs with groff and mom.
This is more of an annoyance than a bug, and may safely be ignored.