How to print a drawing to a defined scale (e.g. 1:2500) in ArcMap, and to a given paper size (say A3)?
- Set your Page/Print settings - File > Page and Print Setup...
- Select your printer and page/paper size
- Switch your ArcMap view to Layout using the buttons on the bottom left of the scroll bar or View > Layout View
- Set your Scale to 1:2500
- Adjust your layout if required
To Export to PDF instead of printing use File > Export Map..., change your Save As type to PDF, give it a name/location, and hit Save.
You may need to play around with the PDF options to get the file right (there's Options at bottom left of the Export Map window) - General tab has Resolution and Ouput quality, Format tab has compression settings.
You should first choose appropriate settings in the Page and Print Setup dialog found through the file menu.
Then, make sure you are in layout view and resize the data frame to whatever dimensions you prefer. You can right click on the data frame, mouse over distribute, and then click Fit to Margins if you choose. Lastly, select the data frame while in layout view and simply change the scale as you would normally do in data frame view (just type your desired scale in the box in the main toolbar). You may have to pan around a little bit to get the correct extent.