You can use the LEFT$ function to return a string with a specified amount of characters from the left of another specified string.
That means you can do something like
A$="XDF" 'START WITH XDF
INC A$,"PDF" 'CONCATENATE PDF
A$=LEFT$(A$,LEN(A$)-2) 'EXTRACTS A STRING FROM A$ THAT'S THE LENGTH OF A$ MINUS 2 (REMOVES LAST 2 LETTERS)
1そうだね プレイ済み