Downsample pdfs

Question

I have a 24"x36" poster with a 300DPI background image wieghing in at around 180. If I export a PDF it comes out at 300MB. I have no image compression or downsampling - yet.
If I scale the image down to 50% and add:
colorBitmapSampling = Sampling.BICUBIC_DOWNSAMPLE;
colorBitmapSamplingDPI = 300;
to my pdf export, I should get a smaller PDF correct? I don't really want to jpeg the images - what other tricks can I use to keep the file size down? I think I have all the pdf export parameters but not sure
thanks,
Bob
function myPDFExport()
        //Basic PDF output options.
        pageRange = PageRange.allPages;
        acrobatCompatibility = AcrobatCompatibility.acrobat8;
        exportGuidesAndGrids = false;
        exportLayers = false;
        exportNonPrintingObjects = false;
        exportReaderSpreads = false;
        generateThumbnails = false;
        try
            ignoreSpreadOverrides = false;
        catch(e) {}
        includeBookmarks = false;
        includeHyperlinks = false;
        includeICCProfiles = true;
        includeSlugWithPDF = true;
        includeStructure = false;
        interactiveElementsOption = InteractiveElementsOptions.doNotInclude;
        //Setting subsetFontsBelow to zero disallows font subsetting;
        //set subsetFontsBelow to some other value to use font subsetting.
        subsetFontsBelow = 0;
        //Bitmap compression/sampling/quality options.
        colorBitmapCompression = BitmapCompression.none;
        //colorBitmapQuality = CompressionQuality.eightBit;
        //colorBitmapSampling = Sampling.none;
        colorBitmapSampling = Sampling.BICUBIC_DOWNSAMPLE;
        colorBitmapSamplingDPI = 300;
        //thresholdToCompressColor is not needed in this example.
        //colorBitmapSamplingDPI is not needed when colorBitmapSampling
        //is set to none.
        grayscaleBitmapCompression = BitmapCompression.none;
        //grayscaleBitmapQuality = CompressionQuality.eightBit;
        grayscaleBitmapSampling = Sampling.none;
        //thresholdToCompressGray is not needed in this example.
        //grayscaleBitmapSamplingDPI is not needed when grayscaleBitmapSampling
        //is set to none.
        monochromeBitmapCompression = BitmapCompression.none;
        monochromeBitmapSampling = Sampling.none;
        //thresholdToCompressMonochrome is not needed in this example.
        //monochromeBitmapSamplingDPI is not needed when
        //monochromeBitmapSampling is set to none.
        //Other compression options.
        compressionType = PDFCompressionType.compressNone;
        compressTextAndLineArt = true;
        cropImagesToFrames = true;
        optimizePDF = true;
        colorTileSize = 128;
        grayTileSize = 128;
        cropMarks = false;
        omitBitmaps = false;
        omitEPS = false;
        omitPDF = false;
        pageInformationMarks = false;
        pdfColorSpace = PDFColorSpace.unchangedColorSpace;
        //Default mark type.
        pdfMarkType = 1147563124;
        printerMarkWeight = PDFMarkWeight.p125pt;
        registrationMarks = false;

Answer 1

Yes, if you're scaling, you definitely ought to downsample images during export. I don't know your reasons for avoiding jpeg compression, but that would give you the greatest reduction in file size. (Maximum quality jpeg compression is not  visually detectable to me at all, except maybe in large, subtle gradations.) If you're not going to use jpeg compression on the images, I really don't see any reason you wouldn't at least use zip compression. You can also use
compressionType = PDFCompressionType.COMPRESS_OBJECTS
to compress the pdf itself; I can't say I understand exactly what that does, but I've never run into a problem with it. You could also exclude ICC profiles, depending on the needs of whoever is using the file downstream.
By the way, it looks to me like your function is just setting a bunch of global variables. Maybe there was a
with (app.pdfExportPreferences) {
in there at some point?
Jeff