When I pdf an excel table, two borders that are exactly the same appear different. If you zoom way in, you can see they are exactly the same, but when you zoom to 100%, one looks darker than the other.
Create PDFs, Edit and convert PDFs, General troubleshooting
What is the version of the Adobe Acrobat DC you are using? To check the version got to Help > About Acrobat and make sure you have the latest version 21.01.20149 installed. Go to Help > Check for Updates and reboot the computer once.
Also, please try the following preference setting: Go to Edit (Win), Adobe Acrobat (Mac) > Preferences > Page display > Under rendering , select 'Enhance Thin Lines' > Click OK and check.