As always, cite the version of the work you are using.

Let’s say you wish to cite the libretto of the opera Dialogues des Carmélites, accessed as a PDF. The opera was composed by Francis Poulenc, and he adapted the libretto from a play by Georges Bernanos. The text of the libretto you are citing was translated and revised by Joseph Machlis. 

To cite this source, follow the MLA format template. List Poulenc as the author and follow his name with the labels “composer” and “librettist.” Then list the name of the libretto, in whatever form it is given. In this case, though the opera is called Dialogues des Carmélites, the libretto is titled The Carmelites, so that is the title you should list in your entry. In the Contributor element, list Joseph Machlis as the translator and reviser. Then list the publication details given in the text, followed by the URL for the PDF:

Poulenc, Francis, composer and librettist. The Carmelites. Translated and revised by Joseph Machlis, Peter Moores Foundation, 30 July 2006, www.chandos.net/chanimages/Booklets/CH3134.pdf.

Note that Bernanos’s name does not appear in the entry. This entry is for the libretto, not for the play from which the libretto was derived. Since citations in MLA style avoid providing publication history, the works-cited-list entry provides information only for the source cited, not for an earlier version of the work.