The answer is ... it depends.
If you are viewing a PDF document using Adobe Acrobat Reader, then that is a piece of software designed to view PDFs, not edit them.
The basic choices are -
1. Buy a copy of the full Acrobat software, which will allow you to edit and create PDFs
2. Highlight the text, copy the paste it into a text editor (e.g. Notepad or Word)
3. Obtain a shareware / freeware PDF creator
4. Visit the Adobe website, use their free online PDF software to convert it to something like HTML
Bear in mind that the original author of the PDF document has the ability to set restrictions on the document (e.g. no copy, read only, digital watermark, etc, etc)