GoogleGuy says that some Google engineers may have made some crawling changes that may result in less PDF, DOC, and Amazon pages, compared to recent observations.
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Hey, I wanted to thank everyone for the discussions we had about pdf, doc, and some commercial pages--I know it took some time to collect more data on this issue. I believe that a couple crawl engineers did some in-depth digging for what they could improve. I think it might take a few more days for the crawling changes to fully propagate, but I kept a test query of my own, and the level of pdf/doc/etc results looks more normal to me now.