Sorry for the delayed response and inconvenience caused. As per the description above, the simplified Chinese Font changes into Traditional Chinese font when you copy the fonts, Is that correct?
Sometimes, when a PDF is created by an application and that application does not interpret the specifications of PDF, you get the error message.
If you have Acrobat Pro, try running Preflight to list syntax errors in the PDF file and then run the Preflight fix-up.
And the correct place in your workflow to embed fonts is when you create the PDF file, not afterward in Acrobat, for detailed information, you can refer to the discussion and the answer marked as correct How do i enbed fonts in acrobat dc pro?
You can also refer to PDF fonts, Adobe Acrobat
Is it possible to share the sample PDF file to us so that we can test it at our end? To share the file, please use Adobe Send feature, upload the file, share the link to files via private message only, How Do I Send Private Message
Let us know how it goes and share your findings.
If the font is not available, Acrobat may have no way to know which language applied, because it's the same Unicode point for tens of thousands of common characters. This is called "Han unification" You would not be the first to observe that this was not an ideal decision by the designers of Unicode. Han unification - Wikipedia . You may be able to change the font after pasting.