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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

* Advent won't withdraw offer after deadline-sources (Adds sources on Advent bid)

FRANKFURT, Feb 26 (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.

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

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 a more than 5 percent stake, called a day later for the firm's boards to run a non-biased, transparent sales process.

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 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.

Advent has given Stada's management until Monday to respond to its offer, but two people familiar with the matter said the group would not withdraw its offer once the deadline expires, adding it would participate in the structured bidding process.

Advent declined to comment.

($1 = 0.9471 euros) (Reporting by Christoph Steitz and Alexander Huebner; Editing by David Clarke and Mark Potter)