Letters showing dark and others light

[Ben27992774vjf0]

Hi,

Im using AutoCad and producing PDFs, The text on my PDF seems to show very weirdly, Some letter in a paragragh of text will be dark and normal and others will be a light gray and difficult to read? 

Any help and advice would be great, I know its not the text itself as i can review the same document in a different PDF software without issue?

Many Thanks,

Ben