All of our new high school Science textbooks come with a digital edition of the book, available on a CD or the textbook website. These are great to have on your school computer, and perhaps your home computer. Not only can you copy text from these pdf files, but you can also make use of any tables or graphics in your class presentations and such.
I'll assume that you're using the official Adobe Acrobat Reader, even though I recommend Foxit Reader. The process is basically the same for for any pdf reader, to copy text just select the text (you may have to first choose the text select tool or the select tool). You can then paste it into whatever other document you are working on.
To copy a graphic of any sort you want the snapshot tool. With the snapshot tool you just put a box around whatever you want to copy and then paste it into your document. However I would recommend zooming in on the part you want to copy first, because the zoom level of the Acrobat Reader affects the resolution of the copied image.
Note that these tools should certainly not be used to violate copyright law.