Folder "ViewerFiles" takes 1/3 of disk storage - can older files get removed?

Hi all,

we noticed that the folder "ViewerFiles" takes 1/3 of the disk space at one customer. As far as I understand, these are just preview files.

Can we delete all files that are older than a year? Or would this lead to errors in the client?
Does anybody have experience?

Thanks,
 Beat