To know how to use all the functionalities of the Supplier portal, have a look at this article: http://help.lsp.expert/en/articles/3340222-getting-started-with-the-supplier-portal
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To unshare a file you shared with a supplier, you have two options:

1) In the Files tab of the job, it the file is shared with all your suppliers, you can click on the three dots at the end of the line of the file and then click on Remove sharing with all suppliers.

2) In the "Files shared with the supplier" panel in the outsourcing page of the job, you can click on Unshare next to each file.

To know how to use all the functionalities of the Supplier portal, have a look at this article: http://help.lsp.expert/en/articles/3340222-getting-started-with-the-supplier-portal
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To share a file with a supplier, you clicked on "Manage your files" in the Supplier portal (Outsourcing tab of the job), you selected the file in question and you clicked on the dark blue eye button next to the corresponding file:
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A green eye appeared on the left of the file, to indicate that the file has been shared with the supplier. The eye button on the right was then replaced by a red eye button, on which you can now click to unshare the file.
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Good to know: you can also see (but only see, not modify) which files have been shared with the supplier in the Files tab of the Job.