That depends on what you mean by solid backlinks. If you simply mean you want a lot of them, then spam away to every free distribution site in creation like most folks who have no idea how to use press releases most effectively. If you mean, how can you maximize exposure in legitimate sources who will in turn provide you with fewer, but better backlinks (permanent, relevant, from authority sites, etc.), then target your distribution to influencers of the end target audience you're trying to reach. There is absolutely NO single distribution outlet or method that will work in the best possible way for every release. I have several articles on the subject that may be of interest: Effective Free Press Release Distribution in 5 Easy Steps 7 Signs That Your Press Release Sucks What Press Releases Aren't Get High Quality Backlinks with Press Releases
PR itself is a solid submission process... PR are something related to news and latest happening in you area or company or something... best places for getting more solid backlinks could be news submission portals like Google News, TOI etc..
PR submission is solid submission process, you get more media coverage and exposure, drive more traffic to your site.