hdd file decrease

Take backup :-) - one way to do it

Shrink your file system(s) to the smallest possible size
Shrink your partition(s) to the smallest possible size
Shrink yuur image (raw, lvm) to your new wished size
Extend your partition
Extend your filesystem(s)

It is similar to the extending disk

http://pve.proxmox.com/wiki/Resizing_disks

But in opposite direction.