How are PDF sizes specified?

PDF's inherently a print medium, and its internal coordinates work in terms of 'points' (72pts per inch). The PDF rendering software (Acrobat, FoxIt, Ghostscript, etc...) will query the output device for its DPI rating and internally convert all the point-basec coordinates into device-specific pixel sizes when it comes time to render the PDF for display/print.

You can specify sizes in pixels while building a PDF, certainly. But remember that pixel sizes differ. A 300x300 pixel image will be 1" x 1" square on a 300dpi printer, but 3" by 3" on a 100 dpi monitor.

PDF internally uses the same graphics model as PostScript. PDF is derived from PostScript. Basically,...

• ...it uses the very same operators that are available in PostScript, but renames them from being long and fully readable to short 1-, 2- or 3-letter abbreviations;
• ...however, it strips all features that make PostScript a full-blown programming language;
• ...and it adds a few new graphics capabilities, such as tranparencies and direct TrueType fonts embedding.

PDF also uses the same basic measurement unit as PostScript: 72 points == 1 inch. You may also use fractions of points. This is the device independent way of stating dimensions.

If you ever use pixels, you can do so. If you do, the absolute size of a graphic object on the display or the printed paper then depends on the current resolution of the display or printer. A square of 72px x 72px is 1inch x 1inch at a 72dpi resolution, but it is 0.1inch x 0.1inch at a 720dpi resolution. Therefore using pixels is a device dependent way of stating dimensions.

A4 dimensions are 'width x height = 595 x 842 pt'.

I sometimes edit my files in GIMP, when I want to edit my pictures and such. When I export for PDF it depends on the quality I need.

I just use this chart (for A4):