Shares plunged as much as 10% in Paris and 7.4% in U.S. trading after Schneider Electric announced it would buy Boston-based industrial software maker PTC for $

Shares plunged as much as 10% in Paris and 7.4% in U.S. trading after Schneider Electric announced it would buy Boston-based industrial software maker PTC for $22.6 billion in cash — its largest acquisition ever, by a wide margin. The market's verdict was swift and punishing, erasing roughly €15 bil…