The main page is probably considered stronger as it has more links (internal and external) pointing to it.
Keywords in links that point to a page are considered part of the page.
So you can
(a) try to strengthen the specific pages by having related pages (reviews / installation guide / comparison etc - put your thinking cap on) that link to them to provide them with more link love within the site.
(b) point more keyword-rich links from outside the site to those pages, and also
(c) point keyword-rich links with the specific page keywords (or part of them) to the main page for a 'trickle down' effect.
Warning: be aware that it's easy to kill your rankings for the keywords you're trying to target by crossing an 'over optimisation' threshold with internal or external anchor text. Although I see people who have hammered away with tightly focused anchor text rocking the SERPs, I've also seen first hand how this can kill your site.