I believe you are referring to one specific pdf or is it happening with all the files you are converting to excel? The pdf file that you are trying to convert in Excel, what type of file is that? Does it actually contains real texts or a scanned page?
Your Excel which was originally created through a pdf would not be editable, if-
1- The pdf file you have converted might be a scanned pdf which doesn't contain any real text.
2- If your original pdf has images rather than text, then after converting to excel document the images would remain the same and that wont be editable.