Hi Fatema,
Welcome to Microsoft Community.
We are so sorry to convey that the Save As feature in Excel Online is not available to export the file from xlsx type to PDF. As there has no out-of-box way to save a workbook as PDF to the SharePoint site. For any suggestions and feedback to improve this demand in Excel Online, kindly submit your ideas in the Feedback channel and let the related product team get informed of this inconvenience. See: How do I give feedback on Microsoft Office?
Besides, if it is available for you to use Excel Desktop App to accomplish the work, then you will be able to use the Save a Copy feature to save the workbook as PDF to a specific SharePoint storage location(even not the same folder). See the screenshot taken from the Excel desktop app.
Also, another thing that may make your work efficient(only if you can use Excel desktop app) is using the Macros, see some threads in other communities for a reference:
Converting to PDF and Saving to SharePoint | MrExcel Message Board
Export Worksheet as PDF to Sharepoint Folder | VBA & Macros | Excel Forum • My Online Training Hub
You can post a thread there to get professional help on creating a related Macro to realize the demand based on your environment.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
Hope this helps a bit and hope a nice day for you!
Best Regards,
Mia