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What size text frame would be used for a document to be viewed in a PDF on an iPad?
You can always zoom in and out on your smartphone or tablet.
In Acrobat you can set the magnification, when the PDF is opened, e.g. Fit Page or Fit Width. However, I do not know, which PDF viewer on the tablet will observe this setting.
When the PDF is opened with Acrobat, there is also an icon like a drop to switch to Liquid Mode. This is a more smartphone-friendly view.
And various models have different sizes and display resolutions.
Therefore I would experiment which size and which zoom factor you need to get your text and graphics in a decent magnification.
I grabbed this screen shot from InDesign—which offers iPad presets—to illustrate what Winfried's answer. There are multiple iPads of varying resolutions:
If you are designing for one of these in particular you can specify the page size in pixels using File > New > Document > Custom.
Otherwise, just aim for the general aspect ratio and know that the user can download a free copy of Adobe Reader and control the view options. I'm highlighting Liquid mode and the View options below in Adobe Reader on my iPad.
Thank you for taking the time for such a complete answer to my question!
Presuming this is classic paged PDF (vs. the nascent Liquid mode), there's actually two questions:
When the end user zooms the PDF to full page on screen, does the page fill the screen without spill, and can they read it?
On AR, classic (US) page size is 8.5×11in., or 1.29:1, or a content region of 7.5×10in with 0.5in margins (classic Academy/SDTV 1.33:1), which matches pretty much no mobile screens in use today. Phones and tablets are all over the place on AR, but mainly at HDTV (1.78:1) and wider (often for no sane reasons).
For a current project that will definitely be PDF and viewed on mobiles, might be printed, but won't be HTML yet, I selected a page size of half-letter (5.5×8.5in) with margins resulting in a content region of 4.25×7.69in, or a resulting content AR of 1.71:1, close to HD. With a body font size of 10pt, a full page should be legible on a lot of phones and larger devices, even those lacking Apple's extra fine pixel pitch.
Thank you for taking the time to give me this information!