Low Res Images when Exporting to PDF

Hi all, I am relatively new to CorelDraw.  My first major project using the software was designing a new brochure for the museum where I work. (I am not a graphic designer/artists...it is just a many hat organization.)  We have CorelDraw and I've been slowly learning how to use it.  I seem to be having a problem upon exporting the file to a PDF, which is very image heavy.  All of the images in the file are at least 300 DPI if not much, much larger.  However, when I export to PDF the image resolutions are very low.  When I sent it to the printer (who does not accept CorelDraw files so it has to be PDF) they remarked upon the low resolution and you can tell the quality is terrible on the proof.  When I print a copy of the brochure on just our color printer at work (same size as the brochure...it is not larger) the images are completely fine.  What am I doing incorrectly?