Investing in high-yield dividend stocks like Pfizer and Annaly Capital could significantly boost your passive income stream. With an average dividend yield of about 9.8%, even a small investment in these stocks could lead to substantial returns. Billionaire investors like Ken Griffin and Israel Englander have bought millions of shares in these companies, attracted by the ultra-high yields they offer. Annaly Capital, a real estate investment trust, boasts a staggering 13.6% yield, attracting funds run by billionaires. However, the company has lowered its dividend by 45.8% since 2019, making it a risky choice for most income-seeking investors despite the potential for positive gains. On the other hand, Pfizer offers a 6.2% yield and has a strong outlook for growth due to its innovative new drugs. With its dividend payout raised for the 15th consecutive year, investing in Pfizer could be a relatively safe way to enhance your passive income.