Can the new owner look through my old HR file without my permission?

Generally, yes, at least to a meaningful degree, and you don't have a veto over it. When a new owner takes over a business and continues employing its staff, it's normal — and often necessary — for basic personnel records like start date, pay history, disciplinary record, and leave history to transfer along with the business, so the new employer can administer your pay, benefits, and entitlements correctly, including the service continuity you're entitled to under the Employment Standards Act, 2000. You don't get to consent to that transfer item by item the way you might for an unrelated third party.

That said, your new employer should still handle what it learns responsibly — using it for legitimate employment purposes, not sharing it more widely than necessary, and being thoughtful about especially sensitive material like medical information, which deserves extra care regardless of who's reviewing the file.

If something in your old file looks inaccurate, or you're worried about how sensitive information is being handled, you can raise it directly with the new employer and ask how your records are kept and who actually has access, rather than assuming there's nothing you can do.