I have to work a lot in my computer so that i don't shut it down very often. But Ubuntu (17.04) slows down after one or two days. I don't run many application. I tried cleaner softwares like Stacer, BleachBit etc but no luck. Restarting PC solves the issue but its a problem for me to restart it.
PC Configuration: Processor: 4x Intel(R) Core(TM) i3-2100 CPU @ 3.10GHz Memory: 8142MB (3157MB used) Operating System: Ubuntu 17.04 OpenGL Renderer: GeForce GT 520/PCIe/SSE2
Rather than file cleaners like BleachBit, what you need is a RAM optimizer. Your slowdown is happening due to garbage accumulation that requires ever-increasing amounts of (relatively slow) swap usage. It's mainly a problem when you're opening and closing programs frequently, and will likely be a bigger problem with software that has a large RAM footprint -- but it will always eventually occur in any system that stays up long enough.
A RAM optimizer will pro-actively find and release RAM blocks that belong to processes that are no longer running, allowing new processes to use those blocks. Most of them also reallocate RAM to move existing blocks to be contiguous, so that the free RAM is all one chunk instead of a lot of little pieces. Processes are supposed to release these blocks before terminating, but it doesn't always happen; this is generically called "memory leakage".
I haven't found it necessary to run a RAM optimizer in my own Ubuntu experience, but I have to restart for kernel updates often enough that garbage buildup isn't a major problem for me. With the problem you report, however, it's likely a RAM optimizer will greatly reduce your slowdown.
free -hcommand output to let us know about it. – Ravexina Jul 11 '17 at 14:58