PDF/X is an ISO defined subset of the PDF standard. The purpose of PDF/X is to facilitate graphics exchange, and it therefore has a series of printing related requirements, which do not apply to standard PDF files. For example all fonts need to be embedded and all images need to be CMYK or spot colors. PDF/X-3 accepts calibrated RGB colors.

The printing conditions or output intent need to be specified in the file. This can be specified in the form of standard profiles using codes, like "CGATS TR 001 SWOP".

In the PDF/X file each graphic gets its own color profile, so even though the file as a whole is CMYK, individual graphics may be RGB (with calibration information).

Active content is not allowed in a PDF/X file. This means that standard PDF features like forms, signatures, comments and embedded sounds and movies are not allowed in PDF/X.

See also:
How to Optimize PDF Files for Web Sites?
How to Compress your PDF files?
PDF Tools Command Line options
PDF Compression Command Line options
JPEG2000 compression in Advanced PDF Tools
Modify Custom Properties in Advanced PDF Tools
Scale your PDF pages with PDF Tools and docPrint
VeryPDF JBIG2 Compression Engine
Understanding "Flavors" of PDF
What is PDF/A? What is PDF/X?
Comparison between JPEG and JPEG 2000