This is a resurrection of this bug: https://forum.obsidian.md/t/selecting-in-pdf-is-very-buggy-and-sporadic/84491
As of 1.8.4 selecting text is better in portrait mode, but still very imprecise and jumpy, though more consistent than it was. Selecting text in landscape mode is still basically not working. As soon as you move down a line the selection jumps to the bottom of the page. (For many pdfs with small font and big margins reading in landscape is necessary).
Here is a recording on 1.8.4, sandbox vault on Android.