Ivan F. Boesky boosted his stake in Zellerbach.

The New York arbitrageur said in a filing with the Securities and Exchange Commission that a group he heads owns 2.3 million common shares, or 8.5%, of the San Francisco-based forest-products concern. A week ago, Boesky said he held 2 million shares, or 7.4%, of the company’s outstanding shares. Crown Zellerbach is fighting a $42.50-a-share conditional tender offer from Anglo-French financier Sir James Goldsmith.