Safe to Delete Keynote Cache under /Library/Container/com.apple.iWork.Keynote/Data/Library/Caches While the question "is it safe to delete /Library/Caches" is commonly seen, OSX also has another path for storing caches (not sym link). Keynote has another cache stored at /Library/Container/com.apple.iWork.Keynote/Data/Library/Caches. In particular, a subfolder named DocumentCaches store a some of my slides created in keynotes. 

Is it safe to delete this folder?
Cache is cache. Deleting it is safe. It's recommended to have all applications involved closed tho. Tools like Onyx are mostly the best way to just do a periodic cleansing every now and then.

If you just intend to delete that cache; no worries. Just close Keynote and remove the caches. You might want to remove the app states for that app too if this is a matter of something behaving weird in while working in it.