Vagulus:
One answer to your question would be that they bleach the hell out of the wood pulp before they make the paper stock. In the case of what you downloaded, maybe they should have bleached the hell out of it after printing. Then, at least you would have ended up with a sheet of white paper. Take a look at many of what used to be good engineering magazines, with some real engineering content, thought and writing; most of them have become filled with cheap advomercials, being passed off a serious technical writing and thought. It seems to be directed at inexperienced people who couldn’t engineer their way out of a paper bag which was open at both ends. Take a look at a good share of the internet, and you see a bunch of crap being written and regurgitated poorly, buy a bunch of dim-whits who want to claim to have been published, but have very slim claim to any real engineering knowledge and experience. You damn-well better read and vet this junk carefully because nobody else is. It’s the real ‘wild west’ in most of the publishing and writing world these days. They have to fill space, not particularly meaningfully, and have member or subscriber numbers to up the price for ad space, that’s all that has meaning to them. There is less and less intent to, or interest in, publishing real meaningful material. Most of it’s free and isn’t worth that.