The Euro Sign is the 20AC in the Unicode, see:

https://www.compart.com/en/unicode/U+20AC

You can add Unicode string with the Oracle UNISTR() function, see:

https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/UNISTR.html#GUID-AAF757DB-6E5D-4548-9E36-6B36BB0BD83E

The basic printing code example:

plpdf.PrintoutText(
    p_x => 20,
    p_y => 30,
    p_txt => 'Hello World ' || unistr('\20AC') || ' !'
    );

Please check the full example and the result: