Don’t export your sharable copy to PDF — print it to PDF.
Exporting creates a PDF that is essentially a master digital copy, ready to be opened in Illustrator or another PDF-savvy app for further editing. All the layers and metadata and other information is intact.
Printing creates a PDF that is essentially what you would get if you printed to a sheet of paper and then scanned it back into the computer. This is also called a “flattened” PDF. It is final output for sharing, not editable output for further editing. Sometimes you will see a PDF named similarly to “Cororate_Report_flat.pdf” and the flat means “this is printout, not a master copy.”
Of course, you will want to keep your Sketch document and ideally also an Exported PDF as master copies for future revisions. But you share the printed one.
If you haven’t printed to PDF before, you go File ▶ Print and then there is a little PDF menu on the print dialog where you can choose “Save as PDF.” That printed PDF will be a lot smaller in file size and won’t have any hidden layers.