replace or remove text on file PDF with sed, dont'work

Question

replace or remove text on file PDF with sed

Good evening, i'm testing with the bash script for to replace text with new text on file PDF, i've resolved with the file exported from file txt to file PDF. But i've downloaded the file PDF on internet for test. i've searched with pdfgrep, for example "Roberto Rossi" and i've already tested with other name "Giuseppe Verdi", don't work. Where i've wronged? I attach the script. Thank and greetings

  #!/bin/bash

function press_enter
{
    echo ""
    echo -n "Premere per continuare"
    read
    clear
}

selection=
until [ "$selection" = "0" ]; do
    echo ""
    echo "PROGRAMMA MENU"
    echo "1 - Sostituire il testo nel file PDF"
    echo "2 - Eliminare il testo nel file PDF"
    echo ""
    echo "0 - Uscire dal programma"
    echo ""
    echo -n "Scegliere il numero: "
    read selection
    echo ""
    case $selection in
        1 ) 
    echo -n "Inserire il testo da cercare: "
    read vecchiotxt
    echo -n "Inserire il testo nuovo da sostituire: "
    read nuovotxt
    pdftk 2010-b4-003.pdf output esportato.pdf uncompress
    sed -e "s/$vecchiotxt/$nuovotxt/g"  modificato2.pdf
    pdftk modificato2.pdf output provaMD2.pdf compress
    echo "Sostituito il testo da $vecchiotxt a $nuovotxt" ;
    enter ;;

        2 ) 
    echo -n "Inserire il testo da cercare: "
        read cercatxt
        pdftk prova.pdf output esportato.pdf uncompress
        sed -e "s/$cercatxt//g"  modificato.pdf
        pdftk modificato.pdf output provaMD.pdf compress
        echo "Eliminati i testi "$cercatxt" nel file PDF" ;
        enter ;;

        0 ) exit ;;
        * ) echo "Please enter 1, 2, or 0"; press_enter
    esac
done