public String getPDFXConformance() {
        return this.getTextProperty(this.prefix + ":GTS_PDFXConformance");
    }
This is somewhat of a hack to handle the older pdfx:
See also the more modern {@link XMPSchemaPDFXId}
tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-pdf-module/src/main/java/org/apache/tika/parser/pdf/xmpschemas/XMPSchemaPDFX.java