The laptop is terribly slowly and I find it very diffcult to use in reading word documents; PDFs etc...
Computer speed doesn't only depent on hardware specs, but on operating system, and your skills and installed software.
For example, I'm currenly writing from my tiny (1kg) laptop which is Pentium 2 (300mhz) w/ 128mb ram. And it's working really fast. But that's because i don' use windows XP (ughhh), as i don't use it on any of my other computers.
1. Is it wise to go for 1 GB each slot so that laptop is having 2 GB RAM...????
depends on your needs. If you really gonna need to use 2gb - than yes. But do you? For example 100 opnened internet sites (in firefox), 20 opened word documents, 10 opened pdf's - all this simultaneously will probably take as much as whole 1gb of ram.What about second gb of ram? - I don't know, you probably need some serious video editing or tremendous database to occupy it.
2. Does RAM of 2 GB will really boost speed of laptop like desktop...????
It depends on frame of reference. You see, If your frame of reference is 1gb of ram, than answer is probably NO. But from the fact that you use Win XP (my guess only) and 256mb ram - definitely YES.
4. I don't know anything about your model in particular, but if your lap uses standardized memory (99% it is) than any ram of this standard and will do, just like in desktop area.
Now, did you even consider a software-related part of your problem solution?
Reguarding operating system - I'm guessing its XP - soooooo. Not too much can be done here. but what about other software?
You said you open pdf documents a lot.
I'll just guess (maybe wrong) that you use acrobat reader for that.
Now if you tried this free pdf reader:
you would discover that it requires 10-20 times!!!! less memory to open same document as adobe reader.
for example opening of 1500-page documet on my pentium 2 300mhz/128mb ram took around 2 secounds.
Same thing might apply to other software that you might be using.