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
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:
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.
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.