PDF/X standards are defined by the International Organization for Standardization (ISO). PDF/X standards apply to graphic content exchange. During PDF conversion, the file that is being processed is checked against the specified standard. If the PDF will not meet the selected ISO standard, a message appears, asking you to choose between canceling the conversion or going ahead with the creation of a non-compliant file. The most widely used standards for a print publishing workflow are several PDF/X formats: PDF/X‑1a, PDF/X‑3, and PDF/X‑4.