Even if you find a legitimate PDF, the quality is often terrible. Many free scans floating around the internet are from 2005:

For millions of gamers worldwide, the name needs no introduction. As the primary character designer and illustrator for the Pokémon franchise, Sugimori is the man who gave visual life to Pikachu, Charizard, Mewtwo, and over 1,000 other creatures. His distinctive watercolor style of the 1990s—soft edges, dynamic poses, and vibrant yet earthy tones—defines the "Golden Era" of Pokémon.

For over three decades, Ken Sugimori has been the driving force behind the visual identity of the Pokémon franchise. As the primary artist and character designer, Sugimori's distinctive style has brought the world of Pokémon to life, captivating fans of all ages with his imaginative and iconic designs. For those seeking to explore the creative genius of Ken Sugimori, a treasure trove of inspiration awaits in the form of his art book, which has been a coveted collector's item for years. Fortunately, enthusiasts can now access a digital version of this masterpiece through the "Ken Sugimori Art Book PDF."

If you are looking for digital or physical collections of his work, these are the most reputable sources: Ken Sugimori Works