If you give the filename `pandoc-word-pagebreak`, pandoc will expect the filter to be an executable in the PATH with just that name. So, if you just have `pandoc-word-pagebreak.hs` in your working directory, that won't work.
Try either `--filter pandoc-word-pagebreak.hs` compile the filter (`ghc --make pandoc-word-pagebreak`) and use `--filter ./pandoc-word-pagebreak`.
Oh, you can test on the command line:
pandoc --filter ./pandoc-word-pagebreak input.md -o output.docx