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You need to locate the 'Acrobat Portable Document Format' mimetype in 'Mimetypes' in SmartSection admin and put a tick the column(s) to give upload permission for users and/or admin. You can then attach a PDF to an article, although AFAIK the default behaviour is for the PDF to download, not open in the browser.
SmartSection (and 'News') can 'make' a PDF using the text and images from an article, which does open in the browser.
If your site visitors are going to be opening PDFs in their browser directly from a list of articles on your site, I suggest that you warn them. There's no 'escape' from the blasted Acrobat plugin once it starts to open a file. It's really annoying if you were expecting a web page.
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