For a poster or publication it may be necessary to submit images with 600 dpi for the best print quality. I use GIMP (download here) to convert pdfs without having to buy Photoshop. GIMP works for any platform.
First import your pdf. Check that the width and height of the pdf is correct, as well as the dpi you want. I’ve selected 600.
Then select the “export” option. Here you can choose any file extension you want. The compression level will affect the final file size.