Search in Preview is not working

I am having problems making the search function of Preview work. There was a previous post that suggested several options to "fix" the problem such as using Command W to shutdown, or switching to Table of Contents view, or exporting the PDF, in order to get the search function to work again. None of these options work for me.

As another user suggested, there seems to be a difference on whether the function works based on the encoding software used to create the PDF. I found this to be true. I have two similar PDFs. One allows the search function to work and the other does not. When I used "Get Info" to see the differences between the two documents, I see that the one that allows searches has the Adobe Acrobat encoding software and the one that does not has MacOS PDFContext. I am running macOS Mojave V10.14.5

Have there been any updates or fixes to this issue that I am not aware of? I have several large documents that it would be helpful to be able to use the search function.

Thanks.