Can only a portion of a longer document like a thesis, proposal or dissertation be reviewed or edited?
But know this: Consistency across a document is only achievable when a complete document is edited!
A complete edit is what could make the difference between an acceptable paper (or not) and an excellently presented paper.
Why you might wonder? Simple.
The editor is taking note that the same formatting and the same language and preferences are being used from chapter to chapter.
Certain crosschecks can only happen if the whole document is available:
Between headings and TOC, and figures/tables and their lists.
Between the in-text citations and the references.
But of course, the choice is always the student’s .
One can certainly start with having individual chapters edited. But at some point, an edit that covers the whole document should be considered.