I've actually abandoned evernote in favor of zim wiki for almost a year now, and have been quite happy. Admittedly, I don't use pdfs, deferring more to text and basic image-style notes. (So, my requirements are admittedly simpler.) Also I haven't synched across devices (yet!) but zim can be set up to simply save the content/notes within a directory that gets auto-synched by another app (such as owncloud, dropbox, etc.)...so I would recommend you try these features of zim wiki before writing something yourself. I think if you gain some direct experience with these features will either motivate you to adjust the zim code to your liking...Or, give you experience to know what direction and specific feature set you would like to code via your own, separate project. Either way its a win for you, and the open source community!