Section 29.4 Page Shape
The latex.geometry switch can be used to supply a single string composed of any options to the \geometry{} command of the LaTeX geometry package, and these will override any defaults supplied by PreTeXt. Resist the temptation to pack in as much text on the page as you can. PreTeXt varies the width of the text in reaction to the font size and is already very close to the maximum number of characters per line for comfortable reading by humans.
An electronic PDF may be printed on physical paper, but perhaps you want to make a version that works well on a portable device that naturally supports a portrait orientation, such as an Android tablet, an iPad, a Kindle (device or application), smart phone, Sony Digital Paper, or a ReMarkable tablet. Aspect ratios vary across these devices, but once you settle on a target ratio, we have had good luck with the following algorithm and parameters:
Specify 10 point text
Text width of about 4.5 inches
Add quarter-inch left/right margins to compute text width
Use aspect ratio to compute an overall height (about 6.5 inches)
Subtract quarter-inch top and bottom margins to obtain text height
Then you can provide the geometry package the overall size as the papersize and the text width and text height as the total size of the body, resulting in equal (tight) margins all around, and good use of limited screen real estate. These parameters create a PDF that is very legible on a larger smart phone, and for fine detail, rotating the device to landscape works well. Really.