It's working fine here. Likely the PDF you're searching, has images & not text in it. You can use OCR software to read the text from the images & convert it to searchable.
Yep, I have seen the behavior myself recently. As a work-around, if you do Command-P to open Print Preview, then click the PDF drop-down at the bottom and choose Open in Preview, it'll create a new file that is text-searchable. So it seems that there are some formats that Preview is having trouble with.