Thanks for using Aspose.Cells for .NET.
It seems your issue occurs because fonts are not embedded by Aspose.Cells. Rather fonts are linked and actually found on user machines.
It seems, Ms-Excel embeds the fonts in the pdf, when fonts are embedded inside the pdf, then these fonts does not need to be installed on user machine.
We have already added a New Feature Request for embedding fonts inside the PDF in our database with the issue id: CELLSNET-41131
However, this feature is not coming soon and could take much time (probably several weeks or months).
As a workaround, I would suggest you to reduce your image size and resolution, you might search on internet how to do this and then embed them in pdf so that you don’t need to use compression library.
Also, there is a huge size difference between BMP and JPG images. If you convert all of your images to JPG format, it could decrease your file size.