Memory upgrade for imac 27inch mid 2011 2.7GHZ INTEL CORE i5
The memory you are talking about (also called RAM) does not "store". It only "temporary" stores. 

It is used for when computer is running to shuffle stuff around getting it from your hard disk (memory) and putting it back to it pending what you are doing. It does not stores anything permanently.

Your files are stored on your hard disk. So go ahead and upgrade the memory.