This is an excellent app for creating navigational structure using table of content entries in pdfs.
I was on the verge of buying PDFPen Pro after a trial period so I could easily create Tables of Contents for big pdf/ebooks. This, however, does the job without distractions, faster, and doesn't crash (unlike PDFpen). It also has an impressive-sounding automatic ToC creation feature that I haven't used (doesn't work on scanned pdfs).
More importantly, if you only want ToC functionality, its much, much cheaper than buying PDFpen pro. Could do with ironing out some GUI tweaks, and a few extra features, but overall, well worth buying.