Hi
I tried to zoom-in and sometimes tried Zoom tools. Whenever I do this, getting an error message saying " cannot do this operation as memory is full" and then it goes black. When I close the pdf it says "closed due to error".

Any one came across this?

Yes. I suspect it is a memory issue likewise, it cannot cope with some large pdfs. I suspect it doesn't use the SD card as a RAM disk and so is limited to its internal memory. I can open pdfs up to around the 22 - 25Mb but it fails on 36Mb.
It will also depend on the rendering it has to do for the document you are trying to zoom into.
On the documents I zoom, I don't have a problem below 150% zoom but do occasionally at or greater than 150%.

I have had it happen as well with a -very- large image file being zoomed to 200%. Just showed the same error window, and a black image.

The same here, 53mb pdf, error message when I tried 150%

Now with a 45mb, i don't have the paging at the bottom of the screen.
PS: They appeared after toggling Full screen view