The next-generation High Efficiency Video Coding (HEVC) standard aims at providing double compression compared to the state-of-the-art H.264/AVC standard. However, this improved compression efficiency accompanies high computational complexity, which is primarily due to the recursive nature of the Coding Tree Unit structure and complex Rate-Distortion (RD) Optimization of Prediction Units (PU). In this paper, we propose a content-driven adaptive complexity reduction scheme that employs an a priori PU size selection algorithm. We adaptively combine smaller PUs into larger PUs depending upon the video frame contents under consideration. Our scheme eliminates the need for recursive or iterative RD cost comparisons to select the best PU size for an HEVC intra encoder. Experimental results demonstrate that the proposed scheme provides significant time savings (on average 43.74 %) for HEVC intra encoding, while incurring an insignificant video quality loss (on average -0.048 dB BD-PSNR).