You can create pdf files from a tif file by using the console program: sips
% sips -s format pdf input_directory/*.tif --out output_directory
ignore the warnings it renames the files for you.
to get more information on sips type:
% man sips
UPDATE: Combine all of the PDFs into a single PDF file:
/System/Library/Automator/Combine\ PDF\ Pages.action/Contents/Resources/join.py -o merged.pdf *.pdf
/mac | Jan 04, 2006