You might want to look into Notebooks (by Alfons Schmid), which can sync with Dropbox, and those Dropbox folders can be indexed in DEVONthink. (No tags.) Notebooks is an excellent iOS companion for DEVONthink, great editor, and plays well with other iOS apps.
Here’s a related tip from Greg Jones for working with Notebooks:
Another, simpler, option is Write, which can also sync with Dropbox.