In this step we will be making a custom Action and then batch processing the pages to add the scale.
Open up one of the pages. Then open the Actions menu in Photoshop and create a new action called "Add Scale" or something like that. It will automatically begin recording so do the following things:
1.) Go to Image->Mode and select RGB Color
2.) Open the scale.png file you just made
3.) Hit Ctrl + A to select all
4.) Hit Ctrl + C to copy
5.) Hit Ctrl + W to close
6.) Hit Ctrl + V to paste into the map page
7.) Shift + V to move the scale to the lower left corner of the map page
8.) Change the blending mode of the scale to Multiply
9.) Ctrl + Shift + Alt + S to save for web as PNG
10.) Ctrl + W to close
Then hit the stop button on the Actions window. You now have an Action saved that will automate the addition of the scale into each page when we now batch process.
Now, close all the windows in Photoshop and go to File->Automate->Batch... This will bring up the Batch menu with a set of options. Make sure the Action you just created is selected in the first drop menu. Then change the source folder to that of your map pages. Then change the destination folder to wherever you want to save the output files. Also change the naming conventions to whatever you see fit. Then hit Ok and the batch command will do its thing, leaving you with a scale on every page!