OpenOffice doesn’t currently let me include PDFs. The following command uses ghostscript to convert the first page of a PDF to a PNG image with a resolution of 300dpi.
gs -q -sDEVICE=png16m -dBATCH -dNOPAUSE -dFirstPage=1 -dLastPage=1 -r300 -sOutputFile=output.png input.pdf
Thanks to a post by gosha bine.