Endnote is primarily intended to help you store citation information and create bibliographies for your academic papers, but it also allows you to collect PDFs of the documents. This is helpful for journal articles, and fantastic for dynamic content.
When reading online articles or web pages that you might need to cite, print to PDF and attach the PDF file to your Endnote record. On Mac OS X, this capability is built-in to the Print dialog. In Windows, you’ll need to install software that allows you to print to PDF.
The purpose of a citation is to allow your readers (and you) to relocate the material you use as evidence in your writing. By keeping a PDF of web pages as you saw them, you have the exact material on which you’re basing your quotations and interpretations. In other words, you (and your critics) have access to the version you’re relying on, even if it’s later changed significantly.
This is particularly important when the material you’re citing changes frequently, such as newspaper articles and political websites.