Try Pdf995. The suite includes PdfEdit995 which allows you to combine pdf
files. It's free if you don't mind ads, or $20 for a suite key to avoid
ads. Pdf995 does a good job creating pdf files. It's what I routinely use.
I haven't used PdfEdit995, though.
Purchase a decent PDF writer such as Adobe distiller this allows you to 'Print' as if directly to a printer. This then deals with the making of the PDF.
I have not tried this, but it may work. Print your VIs using the PostScript option and set your printer to save to file. This will give you a set of PostScript pages. Use GhostScript to assemble them and write them out as a PDF. GhostScript has a command line interface you should be able to automate from LabVIEW. It is also available on several platforms. You can get it here: http://www.cs.wisc.edu/~ghost/doc/AFPL/.