A shareholder's acceptance of the offer is "irrevocable." However, Xerox can back out for any of several dozen reasons.

See the details under "Conditions to the Offer" on pages 58 through 65 of the Xerox Form S-4 Registration Statement, which is located here on the Xerox site: 
(Easy to search the document for words or phrases using F3 or Ctrl-F in your browser.)

However, stockholders are referred by their brokers to a copy of the same document on the site of Broadridge, the company that Xerox has hired to manage the offer. That site is very hard to use. You must step through to pages 64 through 71 in the page-number window at the top of the screen to find the "Conditions" section.
(The search tool on the Broadridge website is very slow and can only handle single-word queries. It cannot search for phrases.)

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