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This page is a collection of valuable sources with information about the Linux operating system and the distributions.
Following are the links to the official documentations for the listed Linux distributions.
Red Hat has a nice overview page for all the documentation.
Main Link: Red Hat documentation
In the main section you will find the links to all the various RH products. The links will send you to an overview page with all versions and links to various types of formats. This would be the Red Hat Enterprise Linux overview link.
You can also use the left side menu to find your product and within in the version. Look out for the small icon next to each document which will allow you to get the different document types including PDF.
As far as I know and understand there is not dedicated documentation for CentOS and you can or have to use the Red Hat documentation.