Finding text in searchable PDF

This may seem an odd question, but I’m wondering how to get find to work with PDF attachments from HeinOnline. The PDFs are fully searchable - if I open the attachment in Acrobat, Ctrl-F allows me to find a word, and I can copy and paste text from it (even from Endnote).

But if I type a word into the find text box on the attachment view page, it tells me the word is not found. Or if I type the word into the general reference search box, the reference does not show up. I think it works fine for “printed” PDF - like from SSRN. But scanned/searchable PDF doesn’t seem to work.

Am I missing something? How do I get this to work?