Thanks for your posting and using Aspose.Cells.
Please first save your workbook in pdf format inside a memory stream and get bytes from it and then save those bytes in azure website directory and see if you are able to save the bytes fine or not.
Please see the following sample code, it gets the workbook in pdf format as array of bytes.
string filePath = @“F:\Shak-Data-RW\Downloads\source.xlsx”;
Workbook workbook = new Workbook(filePath);
//Save pdf in memory stream
MemoryStream ms = new MemoryStream();
//Get the bytes
byte pdfBytes = new byte[ms.Length];
ms.Read(pdfBytes, 0, pdfBytes.Length);
//Write bytes to Azure Website
//Write your own code here, I wrote it to disk to check pdf bytes