Thanks to all for your valuable comments. I've still got one doubt.
The link which I mentioned says : "Group Code 69 = Viewport ID. Changes each time a drawing is opened. Never saved, except for the paper space viewport, which is always 1."
In my drawing file, there are one or multiple viewports in 'Layout1', 'Layout2' tabs. Now I understand that the Viewport ID of each viewport will change each time the drawing is opened. Then which is the Viewport for which the Paper Space ID will remain as 1.