As Colin Jago points out, information on PDF formats and content is typically fairly obtuse. Heaven knows why.
Off I went to read the ISO standards to understand this stuff. When one digs in, there are some fairly simple explanations for this all. I shall try and elaborate.
PDF is now defined in a series of international standards and like all standards, they have to cover a wide range of uses and interpretations. Boiling standards to their essence is quite a skill, one which I get to practice frequently on a professional basis.
Essentially PDF breaks into 2 types - the generic document container and the specific print-ready file.
The general types of PDF (1.3, 1.4, 1.6, 1.7 etc) are wide-ranging descriptions of ways of publishing documents for various applications. Basically the standards define a very general container for a whole lot of data types. Some are application specific (e.g. PDF 1.6 is specifically for engineering data).
In their simplest use (and, I suppose, the one with which most people are familiar) they are just a way of formatting text documents in a way that they can be used across a variety of computer systems. In their most complex form they are multi-dimensional, multi-media applications with hyperlinks, cross-document linking, user input, and a variety of interaction capabilities all done in a way that a variety of computer systems can use them.
The standards lay down a whole lot of requirements on document structure and on the readers that present them to you.
In practice things get muddied by non-compliances, bad formatting and odd things getting embedded. Adobe help the user by providing Acrobat Reader with a pile of capability to try and sort the mess out so that you can use the content.
Then there are the PDF/X types. These are simplifications of the main PDF types specifically for printing and incorporating requirements for colour management data. There are basically 2 types, PDF/X-1 which is CMYK only and PDF/X-3 which also allows RGB colour spaces. The PDF/X standards pretty much says: text & pictures only, no fancy stuff, tell us the fonts & colourspace you're using.
The 2 big differences between the two types of PDF (normal and X-type) are
- PDF/X is a simple, flat file. No layers, hyperlinks, multimedia etc. Just text and graphics for printing on paper.
- PDF/X requires colour management data embedded so that the printer can print things properly.
How does this affect you, the humble photobook writer? In principle, not a great deal. If you're doing a basic photobook and writing a PDF file with embedded fonts, JPEG images with a colourspace and no fancy links, video etc anyway then there is essentially no difference between a PDF 1.3 and a PDF/X-3 file. In fact, there's little difference with PDF 1.4 either (which introduces transparency - if you don't know, you don't need to). Just write a regular PDF and it'll comply with the X-3 format. Or export as an X-3, which should take out any fancy stuff you (or your software) added inadvertently.
I hope that helps.