In Chrome there is not a way to preview PDFs already stored in NetDocuments, and save PDFs from external sites directly to NetDocuments. This is due to the "Download PDF files instead of automatically opening them in Chrome" setting in Chrome.
When this is setting disabled, all PDFs open in Chrome, and the preview pane works for PDFs. When the setting is enabled, the preview pane doesn't allow you to view PDFs, instead PDFs will open in Adobe.
You either have to download the PDF and use the Adobe desktop software to save into NetDocuments, or enable the Adobe Chrome extension, which then gives you an option to open the PDF in Adobe (after you are already viewing it in Chrome), and then it can be saved into NetDocuments.