English is not my first language, so I often need to look up words and this error prevents me from using the Apple dictionary directly. There are many more such errors in PDF files, why does this happen?
Maybe an issue of the PDFkit’s selection & text handling or an issue of the way the PDF documents were created, e.g. this could be OCR errors (probably not in this case). How did you exactly convert the web page?
Not sure if it’s an issue of the WebKit and/or PDFkit framework of macOS but I could also reproduce this by printing the page to a PDF in Safari and opening it in Preview. Unfortunately only Apple can fix this.
If I may, this is usually caused by ligatures in the PDF. A ligature is a single character that represents multiple characters, and is intended to make the text look better.
The usual suspects are ‘ff’, ‘fi’, fl, etc.