How did you export the file?
And also: which version of Illustrator and which system is it?
Preview mode in what application?
It did not show the lines when opened in Corel. In Microsoft Onedrive and Outlook it did not show the lines in preview mode.
Did you check Acrobat?
What did you use for those lines, a transparency effect, blend mode, opacity mask?
Not all PDF viewers support all the features possible in PDF.
The lines show in Acrobat. It was just a simple line tool.
How thin are the lines?
That was the problem, thank you!
Good to hear that helped.