I use Preview on mac OS X to view and comment on pdfs. One thing that bothers me most is bookmarking. The bookmarks added in Preview can not be recognized by other applications I use, including PDF Expert on iPad and Adobe Acrobat, which could see the bookmarks generated by each other with no problem. Oddly, the bookmarks added in these 2 applications can be seen under Sidebar/Table of Contents in Preview, but not under Bookmark tab. After all, I can't stop reading and leave a bookmark in Preview, then pick it up on iPad or other computers that only have Acrobat. Any solution? Thanks.