Either open the PDF directly in your graphics software or right-click on the PDF's filename, select “Open with...”, and select the name of your graphics software.
Then, using the tools in a standalone graphics software (Adobe Photoshop, Gimp, etc.):
Set resolution to 300-600 ppi.
Set bit depth to 8 bit.
Crop unnecessary white space, leaving a fine (2 point) border.
Size the image with proportions constrained, using recommended Dimensions.
Remove alpha channels using the channels or transparency tools.
Flatten the image.
Save the file, using LZW compression.
In GIMP, do not save-as; use Export and select TIFF as the format.
In Photoshop, select LZW compression and Discard Layers, and Save a Copy.
