I know only Javascript and would like to convert a large numbers of scanned documents into layers to mount on white a4 canvas

Question

I have got more than 400 scanned documents of A4 size. But they are not of uniform size. Luckily they are all of 300 dpi.They were created on a scanner. When opened in photoshop they contain random patches that are transparencies, although there seems to be no difficulty viewing themon adobe reader.
My laptop had "Gb RAM and does not accept CS4 I have Photoshop CS3. My javascript and photoshop are both limited to basic skills.
I would like to mount each document on a white A4 sized photoshop background of300dpi, as a layer, flatten it out and save as the desired page.
As this is rather time-consuming, I would like to have a script that will perform this task, on a single file.
I would like to manually run this script on each file.
If possible I would additionally like to  put the individual pages in a folder and run the script repetetively on all files in the folder.
Any suggestions or held will be gratefully appreciated
thanks, peter

Answer 1

This has now been modified for single page PDF's.....
#target Photoshop
app.bringToFront;
function main(){
var inFolder = Folder.selectDialog("Please select folder to process");
if(inFolder == null) return;
var startRulerUnits = preferences.rulerUnits;
preferences.rulerUnits = Units.PIXELS;
var pdfOpenOptions = new PDFOpenOptions();
pdfOpenOptions.antialias = false;
pdfOpenOptions.suppressWarnings = true;
pdfOpenOptions.usePageNumber = true;
pdfOpenOptions.mode = OpenDocumentMode.RGB;
pdfOpenOptions.page = 1;
var fileList = inFolder.getFiles(/\.(pdf)$/i);
var FillColour = new SolidColor;
FillColour.rgb.hexValue = 'ffffff';
app.backgroundColor= FillColour;
var outfolder = new Folder(decodeURI(inFolder) + "/Edited");
if (outfolder.exists == false) outfolder.create();
for(var a = 0 ;a < fileList.length; a++){
if(fileList[a] instanceof File){
var file= open(fileList[a],pdfOpenOptions);
var doc = activeDocument;
doc.flatten();
var docname = fileList[a].name.match(/(.*)\.[^\.]+$/)[1];
if(doc.resolution != 300) doc.resizeImage(undefined,undefined,300,ResampleMethod.NONE );
if(doc.width.value > doc.height.value){
app.activeDocument.resizeCanvas(3508, 2480, AnchorPosition.MIDDLECENTER);
}else{
  app.activeDocument.resizeCanvas(2480, 3508, AnchorPosition.MIDDLECENTER);
var saveFile = new File(decodeURI(outfolder) + "/" + docname + ".jpg");
SaveJPEG(saveFile, 8);
activeDocument.close(SaveOptions.DONOTSAVECHANGES) ;
app.preferences.rulerUnits = startRulerUnits;
main();
function SaveJPEG(saveFile, jpegQuality){
jpgSaveOptions = new JPEGSaveOptions();
jpgSaveOptions.embedColorProfile = true;
jpgSaveOptions.formatOptions = FormatOptions.STANDARDBASELINE;
jpgSaveOptions.matte = MatteType.NONE;
jpgSaveOptions.quality = jpegQuality; //1-12
activeDocument.saveAs(saveFile, jpgSaveOptions, true,Extension.LOWERCASE);