The company's latest institutional shareholder has made some troubling allegations against it.There was some intriguing news from a Pfizer (PFE -2.82%) shareholder on Thursday that was not taken well by the market. Confirming media reports, an activist investor has taken a small stake in the healthcare giant, and announced itself with a letter to the company's board. Investors reacted by trading Pfizer stock down by almost 3%; this was a notably steeper fall than the S&P 500 index's 0.2% dip.Starboard confi ...