update: change pdf text parser to pymupdf4llm

[tungsten106]

Using pymupdf4llm instead of pdfminer to parse pdf contents into markdown formats, as suggested by #131.

Pros and Cons:

• pdfminer extract texts only, generated files have no heading, titles, etc. pymupdf4llm, however, could perform a nice markdown featrues including different levels of heading, code blocks and images (could be saved to specific path, but not included in this commit)
• However, pymupdf4llm may easily create lines of digits which belongs to plots, and create non-existing tables. This is a common problem to most PDF parsers, except those using ocr models (such as markers, MinerU).

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[alphaleadership]

maybe this can patch #142

[l-lumin]

I think it's better to let the user choose the engine rather than replacing it

[afourney]

I think it's better to let the user choose the engine rather than replacing it

I agree. There are pros and cons to each. The main thing is to allow a common interface.

Can you propose an interface for this? One option is to just call register_page_converter externally, and the precedence logic would give precedence to whichever converter is registered later. (see here for example: https://github.com/microsoft/markitdown/blob/925c4499f72757abcf6cb521ee10e4844967af3d/src/markitdown/_markitdown.py#L1269C1-L1287C1)

Another option would be to see which dependencies are installed (though this is more opaque)