Sorry for the delay in response.
Could you let us know if you are still experiencing this issue? If yes, please try following troubleshooting steps:
1- Try exporting another pdf file and see if this behavior continues.
2- If using Adobe Reader to export pdf, try exporting pdf online @ this link: https://cloud.acrobat.com/exportpdf or vice-versa.
Feel free to update this discussion if you need any further help.