Music sheets exported as PDF documents can be read by some music readers and in some cases even recovered again to their original notation forms. In the case of pdf to mxl conversion, you can use for example SharpEye Music Reader that should be able to perform this music sheet data conversion.
Convert Adobe Portable document format to Compressed MusicXML music notation file.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert pdf to mxl on OS X
A music scanning and notation program
Convert from pdf file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.