I am not a lawyer and this is not legal advice.
First, check any license terms that accompany Company S's documentation. They might have published it with the intention that other vendors will incorporate it (e.g. some Apache platforms), or they might not intend that but allow it under their license (e.g. Stack Exchange, or anything else that uses the CC-BY-SA license). Be sure to check whether any license you find permits commercial use; some don't.
In the absence of any explicit permission or license, incorporating their documentation into your own would be making a derivative work that might violate their copyright. (Wholesale incorporation would very likely be a copyright violation; being selective would come down to a question of how much and what you selected, and the limits there probably vary by jurisdiction.)
If Company S's license permits incorporating their documentation into other products, check the license to see if attribution is also required. Even if it is not, in my experience (technical writer in the software field) it's best to attribute anyway. Not only is it more honest, but it's also visibly honest -- important if you're concerned that people who don't know that you had permission notice the copying. You don't want to get a reputation as an unscrupulous business.