I don’t split my large documents, PDF or other filetype.
Instead, when I’m making notes, I do that in rich text notes that can link to the source document. I usually use an Item Link to link to the source and, in the case of PDFs, may also use a Page Link to refer to a specific page, e.g., the page location of an extract. I may use other tricks to link to a specific area of a document, PDF or other filetype, such as a text string that can be used in a Lookup (Command-/) search.
The ‘Annotation’ smart template (Data > New From Template > Annotation) is a convenient way to start a note about a source document, as it is automatically linked to the source. Often, I’ll use DEVONnote for note-taking, as it can be set up as a floating window that can be minimized to the Dock when not in use; a DEVONnote rich text note can then be clipped or copied/pasted into a DEVONthink database.
Some notes may refer to a single source document, others may refer to multiple sources about a specific topic. I might, for example, use See Also or See Related Text to explore my database for similar (contextually related) items, and broaden the note by referring to (and linking to) other useful documents. Suppose I wish to see all the documents that contain a term, such as ‘tort’; Option-click on ‘tort’ and a list pops out on the right. Or suppose I wish to see if other documents contain a particular phrase, “biological diversity”; select the phrase, Control-click, choose Search Selected Phrase and a list pops out on the right.
Tip: Usually, when using one of the above tools to explore terms and concepts I don’t want to lose my place in the document I’m viewing. So I can open a document suggested by See Also or Search Selected Phrase, for example, by Control-clicking on one that looks interesting and choosing the contextual menu option, Open in Tabs. Now I can switch to the tab holding that document, perhaps copy a quotation from it, return to the tab holding my note in progress and paste in the quote, then go back to the tabbed document, choose Edit > Copy Item Link and paste that back into my note, so that I’ve got a link to the source. (That link to the source of the quotation will come in handy if I wish to footnote the quote.)
I mention these tools and techniques to illustrate two points:
There are other useful tools in DEVONthink, in addition to smart groups, that allow one to quickly aggregate documents that contain desired information (Tags offer still another tool, and there are others available in the Search window); and
Creation of such searchable notes in the database can serve as an alternative to splitting large documents, as the user is in effect creating new summaries and overviews of them, especially for the user’s primary interests in their content.
And a third point, gratis: I’m still awed by the tools that Christian and Eric have designed in DEVONthink to help me explore information in my databases. When I’m fishing for an idea or have hit a writer’s block, DEVONthink almost always saves the day.