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Yeah the second batch of sites I listed have been around, or at least displacing our pages, since probably July of last year. I'm waiting for them to be penalised and disappear, but they've recently updated their layout with a responsive design and seem to still be going strong.

A short product description on my sites is about 100 words and a unique photo with a generous caption - so about 150 words total. Most products get a much longer description of at least 300 to 500 words and at least one unique photo.  The short description described above is often upgraded after we receive a few questions about a product or after we get more experience using the product ourselves (we have experience using almost every product that we sell).  When the descriptions are improved, the rankings often go up and the page starts pulling in traffic for long tail keywords.  These are the two benefits that we see from improving product descriptions. Better selling products and those that receive lots of questions can have even longer descriptions of 500 to 1000 words and more photos.  Best selling products also usually have separate articles that explain how to select the product, how to use them, how to maintain/repair and more.  We always have links to these articles within product descriptions.  Our product pages and article pages often rank #1 and #2 in the SERPs.  We also have lots of #1, #2, #3 positions as a result of our article pages. These really long descriptions usually have the essential info in the first one or two paragraphs.  Information after that has subheading to make it easy for shoppers to scan and skip/read parts of the description that they want. If we have a product that gets a few returns we often add information about the product that caused people to return it.  We would rather kill a few sales than incur costly returns.

Yes, you absolutely should add unique text to each of these pages. Not only so that they aren't flagged as duplicate, but because it's always an SEO benefit to have more good content. If you don't have the capacity to write such content, however, you may want to remove them from indexation. The reason that these pages are being flagged as duplicates is that Google isn't parsing these PDFs. Which means that, all Google and others see are pages with no content and an iframe. It's also pertinent to note that Moz will flag anything with more than 90% overlap as a duplication. I hope this helps!