Below is the result of converting a PDF with unknown or undefined unicodes. You will see that each character in the text box has been replaced with a tilde:
After selecting the “Edit Unknown Characters” Preference and converting the PDF again, the Characters Editor will come up and then each Character Cell can been “Edited” until all cells are set to a green “Edited” status:
Upon clicking the “Finished” button, the conversion results in the text using the edited unicodes:
Therefore, the accuracy of the text conversion will depend upon the thoroughness and accuracy of your editing.