The feature is there. I assume our code is still used, but it may have been rewritten since I retired. You can use this as an example if you like: https://www.extension.purdue.edu/extmedia/FNR/FNR-269-W.pdf

(which is random, but you need two columns in the PDF). Try select from column 1 near the bottom, across to column 2.

PDFs are very diverse. Our algorithm does not always work, alas, but it is significantly better than what existed before.

In fact, for some reason, it works well for this document on page 1 and 3 but not so well on page 2! But I no longer have access to the low level tools I used to use to figure out what went wrong.

This is so neat. Like someone else mentioned already, this feature has been very useful for me countless times. In fact, when it doesn't work, I tend to blame the PDF itself :). Thanks a ton!

Yup, just open Preview for an image (possibly other documents as well e.g., PDF), go into “markup” mode, and begin to draw with the pen or pencil. Preview will try to “perfect” your drawn check mark, X, straight line, curved line, etc.

That's a nice feature! But it was added after we left Apple. I cannot claim any credit for it.

That's a different "smart" feature -- the one mentioned has to do with guessing where the selected text continues (when it goes on to a different column, text box, etc).

PDFs are more "baked" layouts rather than high level semantic markup, so it's not easy to automatically determine this. The AI is probably a set of heuristics.