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PDF 1.4 and later files support live transparency, but until the PDF/X-4:2007 standard, PDF/X files did not. Thus, you can save a PDF 1.4 file from InDesign, Illustrator, or Photoshop, and any transparency remains live; but the resulting PDF 1.4 file is not a valid PDF/X file. For this reason, CS3 components create PDF/X files in PDF 1.3 format, as valid PDF/X files, with properly flattened transparency.
There is absolutely nothing wrong with the level of PDF generated by InDesign 7.0.x (CS5) for the PDF/X standards.
PDF/X-1a:2003 allows for PDF 1.4 without live transparency, but doesn't require PDF 1.4. PDF 1.3 does suffice and the PDF/X-1a:2003 generated by InDesign does fully comply with the standard. In other words, PDF/X-1a:2003 absolutely does not require PDF files so-tagged to be also tagged as PDF 1.4. The exact same issue applies to PDF/X-3:2003 versus PDF/X-3:2001.
With regards to PDF/X-4:2008, the PDF 1.4 files with live transparency exported by InDesign 7.0.x (CS5) and 6.0.x (CS4) fully comply with the PDF/X-4:2008 standard. PDF/X-4:2008 allows features in PDF 1.5 and PDF 1.6 that don't exist in PDF 1.4 such as JPEG 2000 raster image compression, layers, and more highly-compressed object streams, but it doesn't require there use of that the file be tagged as PDF 1.6.
InDesign 7.5.x (CS5.5) does support PDF/X-4:2010 including the additional features of PDF 1.5 and 1.6 that are not in PDF 1.4 or supported by earlier versions of InDesign.
- Dov (chair of ISO TC130 WG2/TF2 PDF/X)
There is no such standard as PDF/X-4:2007. The first approved PDF/X-4 standard was PDF/X-4:2008 which was first supported by InDesign 6.0.x (CS4). PDF/X-4:2008 was totally replaced by PDF/X-4:2010, supported by InDesign 7.5.x (CS5.5) in 2010.
Note that all valid PDF/X-4:2008 files qualify and validate as valid PDF/X-4:2010 files. PDF/X-4:2010 provides less stringent rules for support of layers. InDesign 7.5 supports layer export for PDF/X-4:2010 as well as the other PDF 1.5 and PDF 1.6 features supported by PDF/X-4 in PDF 1.6.Note that InDesign 5.x (CS3) supported export of PDF/X-4 (Draft) based on a late, but not final draft specification of PDF/X-4. Such files for the most part are absolutely not valid PDF/X-4 files, failing validation for same with Acrobat Pro or any other proper preflight software!- Dov