Hi Piotr,
Thank you very much for using Syncfusion products.
We are unable to reproduce the reported issue at our side in version 9.23.0.127 (Latest version).
We have created the sample at our side with Excel input file contain Polish Letters and output pdf file generated by Excel to PDF Converter is displayed properly.
Herewith attached the sample for your reference.
Please get the sample and try the sample at your side in version 9.2.0.137 and let us know if these helps you.
The Attached file contains the folder named "Data" is having the
input sample file, Generated PDF file by MSExcel , generated PDF file by Excel to PDF converter.
Please let us know if you require any further clrifications.
Regards,
Sridhar.S
ExceltoPdf_e49f5dda.zip