Perhaps the easiest workaround is to generate an image (jpeg, png, tiff, gif, bmp) of the part of the article you are interested in and insert that image. Many image manipulation software and some pdf viewers will allow you to do this, but the quickest way is perhaps to take a screenshot.

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For posting complete files, you currently have to find your own hosting solution, which unfortunately leads to inevitable link rot. (This used to be the case for images too.) If there is enough interest, there is a chance that SE would try to do something similar to the way they now handle image files.

I'm assuming this is unpublished material. If it is published, either as a preprint or a final paper, then the best solution is to post a doi link or a link to the arxiv. That way, our robots will pick up the link and generate suitable backwards links to your question, when possible. We are currently working on easier methods to insert proper links to published material but we don't expect that to be ready for a few months.