To convert TIFF files to PDF in a program or script, one can use the versatile ImageMagick "convert" program (http://www.imagemagick.org/script/binary-releases.php). To merge three TIFF files, you could use:
convert.exe page1.tiff page2.tiff page3.tiff output.pdf
You can also use it to convert a single multipage TIFF file into a PDF file, but for large files ImageMagick needs a lot of memory and processing might be slow.
