Function fz_open_document 

pub unsafe extern "C" fn fz_open_document(
    ctx: *mut fz_context,
    filename: *const c_char,
) -> *mut fz_document

Open a document file and read its basic structure so pages and
objects can be located. MuPDF will try to repair broken
documents (without actually changing the file contents).

The returned fz_document is used when calling most other
document related functions.

filename: a path to a file as it would be given to open(2).