• It may have a custom font encoding that assigns code points to characters in a way that is incompatible with established encodings such as ASCII or UTF-8/Unicode.

• It may render characters individually out of sequence

• It may have had characters flattened to paths

See Stack Overflow questions How do you debug PDF files? and the now deleted PDF Font encoding — why can't I copy text from a PDF?

To make it text searchable, the best way may be to go back to the original source (e.g. a Word document) and use a different process to produce the PDF. Alternatively you could try rendering your current PDF as a bitmap and then using OCR, but this will be tedious and produce poor results.

I found a way around this problem. I did tools -> edit document text, then for each page, I hit Control-A (select all), then right-clicked and went to properties, and changed the font to something else. After I did this, the text was searchable and I could copy the text!

this might be old but characters encoding issues in compound path pdf are still an issue today I solved by

• open unsearchable text file with illustrator
• Save a Copy as pdf with preset Smallest File Size
• then open file with acrobat
• Scan & OCR > Recognize Text with your settings
• now search ⌘ + f should work

Test source

• compound path unsearchable text file if try copy paste text from this pdf you will get crap

Environment

• sw_vers macOS 14.4.1 (23E224) x86_64
• Adobe Illustrator 24.0.2
• Adobe Acrobat Pro DC Continuous Release | Version 2021.007.20091