What you want to do is apply optical character recognition (OCR) to the images that comprise the PDF. Acorn doesn’t do that, but you can run Acorn’s image file through an open source OCR tool to get the result you want. Tesseract is a command line tool, but it’s very simple to use.
First, you’d need to install Homebrew. Then, in Terminal, you just type
brew install tesseract
tesseract [drop your file here] [drop your file here again but then change the name slightly] pdf
and the new file it generates (with the slightly changed name) will be a PDF with searchable text.
The only downside is that Tesseract doesn’t read PDFs, but it does write them. So your input file can’t be a PDF. If you’re starting with a PDF, you can first export it as a multi-page TIFF with Preview (or it might be possible also with Acorn).