Proofreading is the final step of document completion. You should ensure that your papers have already gone through extensive editing processes before hiring a proofreader (unless you are working with transcripts–or something similar that does not require any substantive editing).
While proofreading, I will look for missing or duplicated words, misspellings, accuracy of punctuation and basic grammar (but extensive grammatical review and rewriting at the paragraph level should have happened during editing), and formatting issues such as consistency of header alignments and extra or missing spaces between words and sentences.
If you have questions about whether your manuscript requires additional editing before it is ready for proofreading, contact me for an evaluation.