This type of issues might occur due to missing fonts. Since Aspose.Cells needs the underlying fonts (used in the workbook) to be installed on the system for rendering to PDF, HTML or image, so you got to make sure all the underlying fonts are there on the pc and your application should access to that folder. You can also find the needed fonts for your workbook using Workbook.getFonts() method. You may put all the font files (.ttf files) in some folder and set the fonts directory at the start before using your original code.
e.g
Sample code:
......
String MyFontDir = "your_fonts_folder_path";
// Setting the fonts folder with setFontFolder method
FontConfigs.setFontFolder(MyFontDir, true);
//.......
//Your code goes here.
//......
I am working as Support developer/ Evangelist at Aspose.