Can i delete large unknown documents?

Do not delete files outside of your user folders, do not delete files from hidden folders, simply put don't delete things that OS X needs (like mobile sync files)

Photos, Music and Videos are your best bet.

Piers1 wrote:

My Mac hard drive is filling up, so I followed the advice from this question

How can I find the largest files on my hard drive (sorted in descending order of size)?

and found that I have quite a large amount of 0.5 GB plus documents with long winded names such as /Library/Application Support/MobileSync/Backup/6edeec7ace74c91eee2957f764c8a36a7b74b470-20140128-190 359

Can anyone tell me what they are, ie do I need 21 of them, and therefore if I can delete them?

Thanks in advance

These are backups of your iDevices (iPhone, iPod, iPad).

You can safely delete them using iTunes prefs > Devices.