Is there any ligature-aware alternative for "pdfgrep" in command line?

I always use "pdfgrep" to search inside of multiple PDF files from the command line. But I met a problem: This ligature character "fi" (see https://www.compart.com/en/unicode/U+FB01).  ; "fi" is in the word "fixed", so I could not search the term "fixed point operator" with pdfgrep -iR 'fixed point operator'. However, when I open the file with PDF readers such as Foxit reader and Evince, "fi" is split into "f" and "i", thus searchable. Is there any more reliable alternative for the "pdfgrep"? Or is there any option keywords in "pdfgrep" to expand the encoding? The PDF file is http://direct.mit.edu/books/chapter-pdf/238450/9780262321037_can.pdf . Ubuntu 20.04, amd64, kernel version Linux 5.6.0-1018-oem. pdfgrep has an option --unac. But if I install pdfgrep with sudo apt-get install pdfgrep, command --unac will report "pdfgrep: UNAC support disabled at compile time!" pdfgrep: Installed: 2.1.2-1build1 Candidate: 2.1.2-1build1 Version table: *** 2.1.2-1build1 500 500 http://mirrors.huaweicloud.com/ubuntu focal/universe amd64 Packages 100 /var/lib/dpkg/status