When you've converted your PDF to Excel, you can edit all elements of your workbook, including the tab name.

However, you can set the tab name before you convert into Excel, but only if you have just Tables and no Fields on your layout.
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The tab takes on the default title "Fields" if there are Fields in your layout. This cannot be changed in PDF2XL - it can only be edited in Excel.
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If you had no Fields and only Tables, the tab would be called "Table 1" (the default name for the first table found in the template), unless you change the table name before the conversion. 

If no Fields are present on a layout, you can change the Table title (and the Excel sheet tab) by right-clicking on the Table, then selecting "Edit Table Title".

Once you convert, you will see that the tab for the Excel worksheet will have the Table Title.
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*To reiterate, if you have a Field in your layout, this tab title cannot be changed from PDF2XL - you would have to rename it in Excel.