Kingbox.

Kingbox. is a completely free and the most advanced file management app.
You can also view and share PDF documents and Office files, save videos and images.
A non-stressful offline viewing is available to you anytime, anywhere.
Features:
-File management (Create folders/Copy/Rename/Protect/Export)
-Document viewing (PDF/Office files)
-Save, view and browse video/audio/image files
-Save Email attachments
-Copy from Camera Roll
-Decompress files (zip / rar)
-Decompress files with password
-Output data to external device　
-High security
[Supported file formats (e.g.,PDF,PPT,XLS,CSV,DOC,TXT,ZIP,RAR,JPG,PNG,GIF,MP4,MP3,etc.)]

-Useful Funections:
-Sort files by date,file format,filename,etc.
-Misoperation prevention during viewer operation
-Zooming In and Out in viewer
-Background playback
-Create your own playlist
-Sleep timer
-Playback speed control

Use cases:
-Carry around files such as proposals and reports
-Decompress files on the go
-Secure management of critical files
-No need to worry about monthly data usage
-Never get stressed by the internet environment
-Listen to music or watch movies offline
-Watch videos during your flight or car ride
-Organize pictures in your camera roll
-Listen to music while working
-Play music you can nod off to sleep to
-Make your own playlist
-Save your audio studying time by speed listening
-Check the movement by slow playback
＼Download the app and try it out／