I recall Matt Cutts, in one of his videos (https://www.youtube.com/watch?v=z07IfCtYbLw), addressing product descriptions (Scraped from the manufacturer) with E-Commerce sites, especially those sites that are selling products that a hundred, or even a thousand other retailers are selling. He said that you would be ahead of your competition if you took the time to rewrite those descriptions, so they are original and not duplicate content . It is not necessary to refresh or rewrite them every couple of months. Just having a unique description that no one else has for that product will put you ahead of everyone else selling that product.

It would be a good idea to try and get your customers to write reviews on your products and post them on your product pages. This is unique, original content that is fresh. Amazon is a good example of doing this.

You want to look at this from what you think your user will be search for. There is a lot of traffic you can obtain through image searches. So if your title and alt tab reflect the search queries, then your images will show high on an image search.

Also, consider pinning your images in Pinterest. I have gotten a lot of traffic through Pinterest for our images, as well as some SEO link value from Google.

There are a couple of things you can do that will help your product pages rank better over your pdf pages. You can do a canonical from the pdf to the product page it is referencing, giving your product page the ranking value. You can incorporate your pdf into text (html) on your product page, giving your product page additional, relevant content, thus boasting ranking.

Need more information to give an adequate answer. There are so many factors that can influence it on both sides of the fence. (Google's side and your side) If you have not changed anything on your site and have not done any off-page SEO, then you are seeing the changes with the algorithm that is affecting your ranking. Google is making changes almost every day, and it will affect your rankings, and those of your competitors if those changes are targeting certain attributes on your pages and other's web pages. As you add more content and pages to your website, it tends to settle down on movement.