Help with Libreoffice Convertion

I use libreoffice under linux to convert an xls or xls to pdf the conversion goes well but in output in the pdf file all the characters are replaced by squares. An idea of ​​the problem ?

exactly these are windows files.
thank you i will try to install ms base fonts

Usually this boxes show, if the used fonts are not available. One solution is to install ms-core fonts in your linux, as your xls files likely originate from Windows.

A work-around is to check if your pdf-viewer can show replacement-fonts for missing ones.

Another solution would be to embedd the used fonts in the pdf.