as long as any images you use have sufficient resolution, are the PDF's dimensions relevant?
PDF files do not have a size in pixels, so if Get Info is giving one that's pure nonsense (and has been for years). They only have a size in inches/mm.
i'm guessing the PDF's 'pixel' is the same as Illustrator's 'pixel' in this instance?
First of all, thanks Doug, and everyone else, for taking time out of your day to answer.
So as I understand it, from what you're saying, and a quick bit of further research, and putting 2 + 2 together of things I already knew...
PDF is a container format. The text is equivalent to a PDF and therefore "vector" and resizable, and the images are output at whatever resolution set by the PDF export preset, and the "reader" (whatever that is in each case) will display accordingly.
That sound about right?
Doug ,Illustrator for some reason allows "pixel" and means 1/72 inch. Many people seem to like it that way, though I think it invites confusion. But for PDFs I think we should take a hard line: "pixel" doesn't mean anything (as applied to a page or document, as opposed to an image on a page): we should be sure to always call it as nonsense, even when Apple are saying it.
zimbop, you have it pretty much except that the resolution in the PDF export preset is a maximum, not a fixed value. It can be less. Each image is different.
I have print documents that are slightly bigger than A4 that the client wants output at retina resolutions (2,048 x 1,536 pixels) to hand on to their app developer. I realise that I could do this as images quite easily, but I believe they want PDF so it is searchable etc.
Both InDesign and Illustrator have a pixel ruler unit (print output measurement). The pixel output unit has nothing to do with image resolution—it is static and equals 1/72". The pixel unit is useful when you are designing for screens where it is assumed that you will have to export the page to an image format. So, I can set up a document as 2,048 x 1,536 pixels (28.44"x21.33") and export it to JPEG at 72ppi and its pixel dimensions will be the same 2048 x 1536 px in Photoshop.
I can also export the page as a PDF and open it in Photoshop at 72ppi and its pixel dimensions will again be 2048 x 1536. The finder info uses the 1/72" pixel ruler unit convention for a PDFs info. I know your oversized A4 is actually 9"x12".
If you need to design for both print and screen you would have to set the aspect ratio of the the print document and do some math in order to export to a specified pixel dimension. If your document is 12x9 the export to JPEG would be 170.666 ppi (2048/12), or your 12"x9" PDF could be opened at 170.666 in order to get the target pixel dimensions. InDesign's pixel unit is there as a convenience so you don't have to do the math for screen only projects.