multiline PDF search

### Checklist

- [x] I have read through the manual page (`man fzf`)
- [x] I have searched through the existing issues
- [ ] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

### Output of `fzf --version`

0.73.1 (Fedora)

### OS

- [x] Linux
- [ ] macOS
- [ ] Windows
- [ ] Etc.

### Shell

- [x] bash
- [ ] zsh
- [ ] fish

### Problem / Steps to reproduce

https://github.com/junegunn/fzf/issues/1183 helps us search inside a PDF, although multiline fuzzy search is the most useful when searching inside PDFs when looking for a specific subject (which can often be reduced to a combination of single topics). An example of this would be asking the question 'How do I plot a point in SymPy?', and for that I would need to fuzzy search the terms `plot` and `point` over multiple lines (as they would need to come from the same paragraph) inside the SymPy documentation.

[sympy-docs-pdf-1.14.0.pdf](https://github.com/user-attachments/files/28914757/sympy-docs-pdf-1.14.0.pdf)

No source file, test, or entry point is named. Start by reading issue #1183 and reviewing how fzf currently receives and searches PDF text; clarify the expected multiline matching behavior using the linked SymPy PDF. Done should include a defined way to search terms across related lines and coverage for the reported example.

Written by the indexing model from the issue text.