What's a good Android app for storing and reading PDF books without taking up much storage?

Keep PDF is a straightforward, lightweight PDF reader built for users who need to manage and read collections of PDFs, like e-books or documentation, directly from their device without relying on cloud services. Its primary strength for this use case is its efficient local file organization. The app creates a dedicated library space, allowing users to import PDFs from device storage or directly download them from browsers. This compartmentalization can help prevent clutter in the main download folder. For reading, it offers standard navigation tools like bookmarks, a jump-to-page function, and a horizontal scrolling view that some find preferable for lengthy documents. A night mode reduces eye strain during extended reading sessions. While it doesn't include a vast suite of advanced annotation tools, basic highlighting and note-taking are supported, which is often sufficient for personal book reading. Reviews indicate users appreciate its simplicity and lack of bloat, which contributes to its relatively small app size and smooth performance on older devices. A limitation noted by some is that the free version displays banner ads, which can be removed through a purchase. The search functionality within documents is present but can be slower with very large, image-heavy files. For anyone building a personal PDF library on their phone—be it for novels, technical manuals, or academic papers—Keep PDF provides a no-fuss, capable environment that prioritizes access and basic utility over complex features.