Alfresco uses SWFTools -- specifically, "pdf2swf" -- to generate Shockwave renditions of PDF documents for document previews. If you're using HomeBrew to manage packages, and you run the following command, you might run into some problems:
$ brew install -v swftools
Namely, the "pdf2swf" command (among others) will not be installed due to some missing dependencies. This is most noticable when installing SWFTools on a clean system. To fix this, install the following packages prior to attempting the above command:
$ brew install freetext libjpeg giflib swftools
This should result in a clean build of SWFTools with the pdf2swf command available in your path and for use with Alfresco.