You can give any document you upload a friendlier display name — an alias — without changing the underlying file. Select the document, then click the Edit Document icon in the document grid header.
In the Edit Document window, the original File name is shown for reference, and you can type a new name in the Document Name field.
When you save your changes, the document grid shows the alias in place of the file name.
Note: An alias is the document's display name for you and other users on your account, including co-counsel who have access to the case — so it isn't private just to you. Deposition participants never see your alias — or the original filename. When you reveal a document, participants see it under a generic revealed name ("Document 1", "Document 2", and so on) based on the order it's revealed. (An original filename can only show through if it's baked into the file itself — for example a spreadsheet that prints its own name in a header.) Your alias and the original filename stay available to you in the document viewer and the Edit Document window.