Steps
Go to Documents
Tick the document you want to split
Click More → Split
In the splitter, pages appear in order. Click the scissors icon between any two pages to start a new document there — a dashed line marks the cut
Optionally:
Drag pages to reorder them
Use Remove page to drop a page
Rotate a page
Rename each output document on the right
Click Send for processing
The original document is archived, and each split-off piece is uploaded as its own document and sent for processing.
Splitting while uploading
From Documents, open the upload menu and select Upload and split PDF. Choose your PDF, then use the same splitter as above.
You can also split a document during a scan, before sending it for processing.
Limits
You can only select one document at a time
The document must be a PDF — other formats need to finish converting to PDF first
If you don't add any cuts, Send for processing will simply process the original document as-is
To join files together instead, use More → Merge.