Knowing what to look for is only half the battle; knowing where to look is equally important. Here are the most reliable sources for finding Japanese reading material in PDF format.
New AI tools are changing the game. You can now use services like Japanese.io or ChatGPT-4 with code interpreter to upload a scanned image of a Japanese page and request it to output a clean, OCR'd, vertical-text PDF. Furthermore, some tools allow you to take any public domain English book and ask the AI to translate it chapter by chapter into simple Japanese, outputting a custom PDF textbook. Japanese Books Pdf