Hey Tahir,
We’ve been also facing the same issue on AWS Lambda, generated PDF document loosing font styles.
Instead of Calibri generated pdf DejavaVuSerifCondensed
Please note that Aspose.Words requires TrueType fonts when rendering document to fixed-page formats (JPEG, PNG, PDF or XPS). You need to install fonts that are used in your document on the machine where you are converting documents to PDF. Please refer to the following articles:
How Aspose.Words Uses True Type Fonts
How to Receive Notification of Missing Fonts and Font Substitution during Rendering