Mendeley, a cloud-based PDF- and file-management tool popular with researchers, has been purchased by Elsevier, the information giant, the companies announced Monday. Mendeley is especially popular with scientists, but it competes with Zotero and Endnote, among others, in helping scholars organize their digital reference materials (it has also added analytics and collaborative tools). While officials of both companies heralded the possibilities of the partnership between the innovative startup and the massive publisher, critics took to Twitter and the blogosphere to express concerns that Mendeley would no longer pursue an open access mission. "The Empire acquires the rebel alliance: Mendeley users revolt against Elsevier takeover," read a headline on paidcontent.org.