One of Pageflow's benefits is the media management. Files are automatically converted into different qualities, to make sure, that stories work on any device and bandwidth.
As soon as you upload a video or an audio file, you are asked to transcode the files. You then automatically receive a notice about how many minutes are being deducted from your account. You can however reuse the media files which are already uploaded, without paying for them again.
Please keep in mind, that the encoding process is causing costs to us. Already used encoding minutes will not be credited to your account, when you delete uploaded files. Thats why we recommend to only upload files, when you are sure they will be used within your story.