Currently, I do not think we can export the Specific Spreadsheet by name into a PDF using the Activity. However, we can specify the Pages to be Exported by Specifying the Start Page and End Page properties. Set Start Page and End Page Properties to 1. It should export only the First Spreadsheet in the Excel file.
An Alternative would be to copy only the Spreadsheet required to a Separate Excel file, and then perform Save Excel as PDF Activity on that file.