I know that you already have an answer that you are satisfied with over on the FB group page, but for the benefit of others who might be reading this thread...

First ensure that you have exported a high quality bitmap of the map. in the "File/Save as..." dialogue you can do this by selecting any of the PNG options available in the dropdown box for file type (png files are better quality than jpg files), then clicking the options button and setting the height and width in pixels to at least 2 or 3 times the height and width of your map in map units. Set the ppi to 300, and antialiasing to about 30-50 %.

Then when you convert to pdf ensure you are using print quality settings - especially the ppi/dpi to 300.