PE investors buy mature companies and extract profits via cost cutting, mergers/acquisitions, re-organizations. A PE firm may buy a paper manufacturing company if they thought they could increase profits by cutting unnecessary costs.

angels/VCs fund growing companies and extract profits as the company value grows and is to sold to a company or the public market. A VC would never fund a paper manufacturing company unless this were a wholly new way of making paper and had potential to transform the entire industry.