Thanks for your response.
I already checked, the same problem exists in adobeviewer also. Moreover adobeviewer has many more problems like white pages, cutting the words on the edges (in fit width mode), etc., forcing to close the book and open again. My first impression is, pdfviewer is excellent for viewing pdf and I also changed my default setting to pdfviewer.
Another problem with both is - in the zoom option, column mode option is not existing. Even PB user manual says, there is a option to zoom for column mode and defining no. of columns. My firmware is the latest 15.2. I am not sure whether this option is missing in this new version but existed in old 14.2 version.
If this is missing only in the latest firmware, please also include this zoom option in the next release. This zoom option will be quite useful in portrait mode.