Kingbox.

Kingbox is a free, advanced file manager that lets you organize folders, rename, protect, and export files. It also views PDFs and Office documents, and saves videos, images, and email attachments for offline access. Decompress ZIP/RAR archives, copy from camera roll, and output data to external devices with strong security and broad format support.

Beyond basics, Kingbox offers practical viewing and playback features: background playback, custom playlists, sleep timers, and playback speed control. Use it to carry proposals, watch videos offline during travel, manage your photo library, and listen to music while you work—all without monthly data usage.