A quick bit of code to generate PDFs from scans, using the iText open-source library.

PDFGenSimple.java

import java.io.IOException;
import java.io.FileOutputStream;
import java.io.File;
import java.io.FilenameFilter;
import java.util.Map;
import java.util.HashMap;
import com.itextpdf.text.Document;
import com.itextpdf.text.DocumentException;
import com.itextpdf.text.Image;
import com.itextpdf.text.Rectangle;
import com.itextpdf.text.pdf.PdfWriter;
import com.itextpdf.text.pdf.PdfTemplate;
import com.itextpdf.text.pdf.PdfContentByte;

// As with all my Gists, this was done quick & dirty for completing the task at hand.

public class PDFGenSimple
{
    public enum ImageType
    {
        PNG, // I might be wrong, but it looks like PDFs support JPG natively, but not PNG
        JPG
    }

    public enum PageLocation
    {
        None, // default for full page scans
        Left,
        Right
    }

    // these get initialized on startup
    protected String inputDirectory;
    protected String baseInputFilename;
    protected String outputFilename;
    protected ImageType imageType;
    protected boolean sideBySide;

    // these get initialized when CreatePDF is called; I won't bother to throw exceptions if accessed before then
    protected Document document;
    protected PdfWriter writer;
    protected Map<String, Image> imageDictionary;

    public PDFGenSimple(String inputDirectory, String baseInputFilename, String outputFilename, ImageType imageType, boolean sideBySide)
    {
        this.inputDirectory = inputDirectory;
        this.baseInputFilename = baseInputFilename;
        this.outputFilename = outputFilename;
        this.imageType = imageType;
        this.sideBySide = sideBySide;
    }

    public void CreatePDF() throws IOException, DocumentException
    {
        this.imageDictionary = new HashMap<String, Image>();

        // PDF setup
        this.document = new Document();
        this.writer = PdfWriter.getInstance(this.document, new FileOutputStream(this.outputFilename));
        this.document.open();

        System.out.println("Beginning file processing...");

        File dir = new File(this.inputDirectory);

        if (dir.isDirectory())
        {
            final String extension = "." + this.imageType.toString().toLowerCase();

            // get a list of all the PNGs in the given directory
            File files[] = dir.listFiles(new FilenameFilter()
            {
                @Override
                public boolean accept(File dir, String name)
                {
                    return name.toLowerCase().endsWith(extension);
                }
            });

            int numPages = files.length;

            System.out.println("Number of pages: " + numPages);

            // for use with sideBySide
            File tempPage = null;

            for(int i = 1; i <= numPages; i++)
            {
                final String filename = GetBaseFilenameFromPageNumber(i, numPages) + extension;

                // try to find the file with the page name provided
                File pageFile[] = dir.listFiles(new FilenameFilter()
                {
                    @Override
                    public boolean accept(File dir, String name)
                    {
                        return name.toLowerCase().equals(filename.toLowerCase());
                    }
                });

                // make sure the results have exactly one file
                if (pageFile.length == 1)
                {
                    System.out.println("Processing " + filename + "...");
                }
                else
                {
                    System.err.println("Count for " + filename + " is incorrect: " + pageFile.length);
                    continue;
                }

                if (this.sideBySide)
                {
                    if (i == 1)
                    {
                        // front cover
                        AddPageToDocument(pageFile[0].getAbsolutePath(), PageLocation.Right);
                    }
                    else if (i == numPages)
                    {
                        // back cover
                        AddPageToDocument(pageFile[0].getAbsolutePath(), PageLocation.Left);
                    }
                    else
                    {
                        // two pages at a time for the rest
                        if (tempPage == null)
                        {
                            tempPage = pageFile[0];
                        }
                        else
                        {
                            AddPagesToDocument(tempPage.getAbsolutePath(), pageFile[0].getAbsolutePath(), PageLocation.Left, PageLocation.Right);
                            tempPage = null;
                        }
                    }
                }
                else
                {
                    AddPageToDocument(pageFile[0].getAbsolutePath(), (i % 2 == 0 ? PageLocation.Left : PageLocation.Right));
                }
            }
        }

        System.out.println("File processing ended!");

        this.document.close();
    }

    protected void AddPageToDocument(String imageFilename, PageLocation pageLocation) throws IOException, DocumentException
    {
        Image img = GetImageForFilename(imageFilename);

        SetupNewPage(img, false);
        AddImageToCurrentPage(img, pageLocation, PageLocation.None, false);
    }

    protected void AddPagesToDocument(String imageFilename1, String imageFilename2, PageLocation pageLocation1, PageLocation pageLocation2) throws IOException, DocumentException
    {
        Image img1 = GetImageForFilename(imageFilename1);
        Image img2 = GetImageForFilename(imageFilename2);

        SetupNewPage(img1, true); //assuming img1 and img2 are the same size
        AddImageToCurrentPage(img1, pageLocation1, PageLocation.Left, true);
        AddImageToCurrentPage(img2, pageLocation2, PageLocation.Right, true);
    }

    protected void SetupNewPage(Image image, boolean sideBySide)
    {
        float width;

        if (sideBySide)
        {
            width = GetSinglePageWidth(image) * 2;
        }
        else
        {
            width = GetSinglePageWidth(image);
        }

        this.document.setPageSize(new Rectangle(0, 0, width, image.getHeight()));
        this.document.newPage();
    }

    // outputPageLocation is ignored if sideBySide is false.
    protected void AddImageToCurrentPage(Image image, PageLocation inputPageLocation, PageLocation outputPageLocation, boolean sideBySide) throws DocumentException
    {
        image.setAbsolutePosition(0, 0);

        float outputX = 0;

        if (sideBySide && outputPageLocation == PageLocation.Right)
        {
            outputX = GetSinglePageWidth(image);
        }

        // this stuff allows you to "crop" images (which really means that a certain part of the image, which is included in full, is rendered)
        PdfContentByte contentByte = this.writer.getDirectContent();
        PdfTemplate template = contentByte.createTemplate(GetSinglePageWidth(image), image.getHeight());
        template.addImage(image);

        contentByte.addTemplate(template, outputX, 0);
    }

    protected float GetSinglePageWidth(Image image)
    {
        return image.getWidth();
    }

    // Caching the images ensures that each scanned double-page of a zine gets added to the PDF only once, halving the file size.
    protected Image GetImageForFilename(String filename) throws IOException, DocumentException
    {
        if (!this.imageDictionary.containsKey(filename))
        {
            Image image = Image.getInstance(filename);
            this.imageDictionary.put(filename, image);
        }

        return this.imageDictionary.get(filename);
    }

    // Pages are 1-indexed. Assumes default Apple scan naming format: BaseFilename, BaseFilename 1, BaseFilename 2, etc.
    protected String GetBaseFilenameFromPageNumber(int pageNumber, int maxPages)
    {
        int actualPage = pageNumber - 1;

        if (actualPage == 0)
        {
            return this.baseInputFilename;
        }
        else
        {
            return this.baseInputFilename + " " + actualPage;
        }
    }

    public static void main(String[] args) throws IOException, DocumentException
    {
        PDFGen pdfGen = new PDFGen(
                "/Directory/To/Scanned/Pages",
                "ScannedPage",
                "/Directory/To/Scanned/Pages/Output.pdf",
                ImageType.PNG,
                true);
        pdfGen.CreatePDF();
    }
}