Hi Andy,
The PDF you shared contains a Type-3 font with custom encoding. However the font resources do not contain a "ToUnicode" table, in the absensce of which we have to guess the encoding. Due to some difference in the way we "guess" during Copy/Paste & Find you are facing this problem. To fix this issue you will have to embed the "ToUnicode" table with the font in the PDF.
Regards,
Praval