I have Laptop having 16GB RAM with normal HDD, I am using windows 8.1 and now it is taking 8GB RAM as cache.
My question is, If I use SSD, will it reduce cache size in RAM as SSD is fast enough to run files directly.
No, it probably won't. RAM is still few orders of magnitude faster than SSDs.
That shouldn't be a problem though, because Windows will automatically adjust cache size when programs demand more RAM. From my experience it will reduce cache size as long as programs use less than 90% of total available RAM, then it will start paging more aggresively. It's possible to allocate over 90% of RAM for programs but Windows will try to avoid that to keep high performance.
In other words, it's good that Windows allocates 8 GB of RAM for cache when RAM usage is low. Unused RAM is wasted RAM, it's better to use excessive memory for cache than leave it unused. If programs ever happen to need more than 8 GB of RAM, Windows will resize cache automatically to satisfy their needs.