Perfection...it's something we all strive for. However, there's a level of subjectivity to perfection. What's perfect in my eyes may be imperfect in another's eyes.
"If everything was perfect, you would never learn and you would never grow."
Will your document be perfect when you hire a proofreader? It depends on your viewpoint!
I feel it's really important to be completely open and honest with everyone when I say that perfection isn't always possible, due to the above reason. You give your file to three different editors and you'll receive three different (mostly) responses.
One reader (we'll call him Barry) will read something and see it as perfect, while another reader (we'll call her Susan) will believe there are errors. Barry thinks you can end a sentence with a preposition, while Susan thinks this is outrageous. The proofreader may allow non-standard past participles in dialogue as it's important to convey a particular regional dialect. Susan is based in Yorkshire and understands this completely, but another reader (Graham?) lives in Australia and believes it's grammatically incorrect. You get the idea.
The writer will provide the editor with a style guide of certain things they want to keep the same, or that they've done intentionally to convey a particular point or meaning. Your editor will (or should!) follow an official style guide too, such as New Hart's Rules or the Chicago Manual of Style.
When I'm proofreading I aim for perfection, obviously! It's what made me become a proofreader in the first place – I'm a perfectionist. However, there are always going to be differences of opinion when it comes to what is right and wrong. To be honest, I'm in numerous groups for proofreaders and there are always debates on the "correct" way to do things.
One person's rules are another person's preferences, as with many things.
It all depends at what point the proofreader gets the file too. In reality, the proofreader should get the document at the very end, after it's already been through rounds and rounds of editing.
In the first instance, the file generally goes through an edit by the writer themselves. It should then go through a developmental edit, which is macro-level and looks at the "big picture" including any plot holes or structural issues.
It should then go to a copyeditor, who will look at any inconsistencies in style and structure. Their aim is to make sure the style is consistent throughout and they will follow the style guide set out by the writer/publisher. The copyeditor will look at the writing on a sentence-level, making sure it flows well and is coherent. After this, it'll go back to the author who will review any suggestions for improvements and the author may then send the document to a layout artist to design the "first proofs".
These "first proofs" are then sent for another edit, generally to a proofreader. At this stage, there will still be mistakes, however there should be a lot less by this point as many mistakes will have already been caught by the copyeditor. While the copyeditor looks at the whole sentence, the proofreader will look at the text letter by letter and space by space on a micro-level.
At this point the document has undergone rounds and rounds of editing, and there are still mistakes to be found.
The document then goes back to the author for them to collate any final amendments suggested by the proofreader in this final stage of editing, and it then returns to the designer to amend the changes for the "second proofs".
The file is reviewed one final time by the author/project manager and it's ready to publish!
PHEW. It's a lot to take in, I know.
Now, if the document comes to me fresh from the author's tightly grasped clutches then it'll have only gone through editing by the person that wrote it. And as I've said before, our brain only sees what it wants to most of the time! If this is the case, as it is 90% of the time, there will be a lot of work to do on the piece, and a higher chance that something may get missed.
As part of my service, I offer a second revision once the document has been formatted by the designer, just to be double-sure that nothing has been missed.
So, what I'm trying to say is that while I try to make sure it's perfect, there will always be a Barry, Susan or Graham that may disagree.