After a few frustrating web searches (most essentially click bait/point to buying another app, or Apple focus that don’t answer my Q), I’m hoping someone here can point me in the right direction.
I want to use an Apple Pencil to highlight text in a PDF using Files (Preview, I assume) and generate the same outcome as on a Mac. As in clear straight lines, not haphazard hand drawn ones.
It was a key reason I could justify getting an iPad, and I assumed (perhaps incorrectly) it would just work… 
You can do it by using PDF Expert on the iPad. It looks like the annotation features are free. Honestly not sure how the app works now, it was a single time fee when I bought it.
I much prefer the UI/UX and simplicity of Preview for what I do (over Acrobat and other apps). And I’d rather not add more apps to clutter up screens beyond what I already have…
For such a simple feature, I can’t understand why it is missing (if it is), given you can do it by finger, albeit text selection first.
Holding the pen on the screen after highlighting the text/drawing the line and it will go straight.
I have angled line ends. My niece who shared the tip with me as rounded ends, haven’t worked out how to change as yet.
I have worked it out.
Rather than using the Markup/Apple Pencil pop up, holding the pencil on a word selects it (and you can drag the handles to select more words/lines). The process then is effectively the same as if using your finger. Select highlight from the context menu that pops up and off you go…
Same outcome as on a Mac.
Turns out, you need to select the tool to the left of the ruler in Markup (then de-select Markup) to be able to select text in this way.
Took me a while to work this out, after being able to highlight by text selecting stopped working
Rather than starting a new thread…
I have updated to iPadOS 17.1.1 recently, and it seems some things have changed with the Apple Pencil.
In the Files app for PDFs, I am now unable to select text for highlighting using the pencil (I can with my finger via long press), and also unable to use the pencil to increase the number of words selected (dragging the handles).
Now, whenever I use the pencil a dotted lined trails: both with and without the annotation toolbar selected-displayed.
Once I have selected text with my finger, and the options are displayed (floating toolbar above the word) I can use the pencil to select options.
In scribble (under settings < pencil) drawing a line or circling a words is still listed, and works in the ‘try scribble’ example.
Any tips on how to get this working again? Maybe a bug?
I had a quick drawing to do yesterday and the pencil did not seem to work correctly. I did not have time to investigate. Your message reassures me that it may not have been user error