I'm new in autocad not have any good idea model and plot viewport etc. My company develop some files from vendors i did some few changes in it. As i checked its plot view there is all model with multiple view port i just want to covert in pdf selected one in A3. Kindly check my screenshot
My first response to you needing your output as a properly concatenated A3 PDF is to state that you should ideally have set up your file with all drawn built environment elements in model space, and though this can vary depending upon draughting standards where you are, for the most part your annotation (notes, callouts, text, dimensions) should be in paper space - that is on the drawing sheet / layout. That drawing layout is primarily how you define a paper / output size, and you place viewports showing model space content with explicitly-set scales onto the sheets / layouts in paperspace. This allows you multiple possible views from one set of drawn elements. It also allows easy set up and control of multiple sheets.
Given you're already out of the gate, which has been left open, and the horse is already long gone - with your file as it is, if you plot, setting a plot range window each time (snapped to your "layouts" that are apparently in model space for consistency) with each being output as a PDF, you can stitch your PDFs together in Acrobat after the fact.
Hope that helps.