Which PDF reader? And did you actually try to print a page or two? What you see on screen and what you get on paper are often two different worlds. So, try printing a page and see how it looks.
Also, a note, for electronic publications, when exporting to PDF (File > Export > Save as PDF) make sure you select "For Screen/Web" in Color tab, "Output intended for" section. Naturally, for printing you'd use "Printer" instead of "For Screen/Web".