what have you tired?
The internal structure of the 2 file types are radically different so unless you know of a conversion function that works on a selected area. Probably not possible.
Have you tired to highlight or select the area to copy and use the right mouse click to see if there is an option to convert to an Excel file?
Note this will not work on form fields in the PDF.
A lot will depend upon how the PDF was created. Unless an Excel file was converted to a PDF using Acrobat's PDF maker, there probably is not enough hidden information in the PDF to convert the PDF context to an Excel file.