I have downloaded texlive-docs using ubuntu repository. How can I view/use them?
Viewing already installed documentation
Assuming you use GNU/Linux, you can view the documentation by typing
texdoc <package name>
into your terminal (e.g.
Viewing online documentation
Go to http://texdoc.net/ and enter the package name in the input field. Choose the documentation that best fits your needs.
Regrading your question in the comments: I assume you use a Debian based GNU/Linux distribution, such as Ubuntu. You can see the texlive-packages enclosed in e.g.
texlive-pictures-doc by entering
apt-cache show texlive-pictures
in your terminal. You will obtain the package description which lists all enlosed texlive-packages. (I omitted
-doc as the description for that doesn't yield a list.)