If a collection has been shared with you, you can view the documents by opening the collection (click the link) and then clicking the name of the document. Then choose Open file. The document is downloaded and accessible. You can also click the file name to download the document directly.
It is currently not possible to view items with a Preview. It is an option, but the function does not work correctly. If you choose the Preview, Brightspace will stop working correctly, after which you have to close the page.