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German drugmaker Stada opens books to rival bidders

* Stada says sets up data room for bidders

* Starts structured bidding process

* Has received takeover proposals from three bidders (Adds context on M&A process, details)

FRANKFURT, Feb 25 (Reuters) - German generic drugmaker Stada Arzneimittel is opening its books to potential acquirers after coming under pressure from its largest shareholder to consider various takeover approaches.

"For this purpose, a data room has been established and the potential bidders - three to date - have been invited to conduct multi-stage due diligence," Stada said in a statement on Saturday.

Stada has become the subject of a bidding war between private equity firms Advent, Cinven and a third group that sources have identified as Bain Capital.

Advent made a binding 3.6 billion euro ($3.8 billion) bid on Thursday and activist investor AOC, Stada's biggest single shareholder with more than 5 percent, called a day later for the firm's boards to run a non-biased, transparent sales process.

Due diligence gives potential buyers access to the target company's books and data.

Cash-rich buyout firms looking to invest in stable healthcare businesses have been working on offers for Stada for months, people familiar with the situation told Reuters this month.

Advent said its offer differed from rivals because it was binding and fully financed, adding that it supported Stada's management and had no plans to break up the business.

Three sources familiar with the matter said that information on Stada's business given to Advent had encouraged the buyout firm to go ahead with the offer.

Cinven and Bain, however, have only received limited data so far and this has prevented them from making firm bids but Advent's move has put Stada under pressure to let others have a more detailed look at its books, the sources said.

($1 = 0.9471 euros) (Reporting by Christoph Steitz; editing by David Clarke)