<p>What is the best way to make the SSRS reporr fit in to PDF page.</p>

Next time please indicate what version of SSRS you are working with...
If you are careful to design your canvas to certain constraints your should be fine. Typically the PDF is going to emulate a 8.5" by 11" piece of paper - considering margins you should keep the width of your report to less than 7" but I think 6.5" is best.
You can do this by selecting the report itself, the menu in BIDS (VS) should now have a report item which you can select to reveal Report Properties. In the report properties you can select the orientation as well as standard paper sizes and margins. Based on your paper size and your left and right margins you should be able to calculate the maximum width for the body of your report. Back in the report designer select the report body and look at the properties. The object drop down list should specifically say BODY, in this you want to ensure the size of your body (width) is not greater than the Report width minus the margins.