First off PageRank is but one of the signals Google looks at, among many other things. If you want "positive SEO", then always focus on quality. Googlebot is smart (most of the time), just because your designing your website with usability in mind, doesn't mean your website will be penalised.
Quite the contrary, your helping your users read a page with heavy text. This is good.
That said. It is true that Google doesn't like hidden text. However this would only damage your site if the hidden text contained spammy text and links. A hidden link to a Viagra website will hurt you. If the hidden text is completely relevant, then by all means go ahead.
I would instead focus on providing a great user experience, and make sure the hidden divs are visible to Googlebot. Meaning use clean code. A good indicator of search engine friendly code is using a text browser to see what a search engine crawler will see when crawling your site.
Lastly, like I said earlier, and as a friendly suggestion, PageRank is but one of the signals. You should widen your grasp of how search works today, specifically with Google. ;D