BRUSSELS, Nov 3 (Reuters) - EU antitrust regulators will decide by Dec. 7 whether to clear U.S. medical device company Thermo Fisher Scientific's $21 billion bid for contract research firm PPD Inc, a European Commission filing showed on Wednesday.

Thermo Fisher, the world's largest maker of scientific instruments, announced the deal in April. The contract research industry is rebounding from pandemic disruption amid growing demand for virtual clinical trials.

The European Commission can clear the deal with or without concessions at the end of its preliminary review or it can open a four-month long investigation if it has serious concerns. (Reporting by Foo Yun Chee Editing by Mark Potter)