If the PDF export option appears disabled, as shown below:
This indicates that you do not have a report selected.
To select or create a report, first go to the Reports tab (second from the right at the bottom):
If you already have a report configured with receipts added to it, tap its name, and then select the Share tab to export its data.
If there are no reports yet, tap the "+" sign to create a report.
After you have created the report, tap its name, and this will take you to the Receipts tab (bottom left):
The report will be empty by default.
To populate it, first tap the report selector:
Uncheck the selected report. This will take you to the list of all available receipts.
Swipe left on any receipt you want to move into your report and tap the move button (orange):
Select the report you want the receipt to be added to.
Once you have added all receipts to your report, go back to the Reports tab and select your report.
To export the report, go to the Share tab (bottom right), select the preferred format, and tap the share button in the bottom middle:
Best Practice: Create the report first, and then adding receipts to it. This way, the receipts will be directly added to the report once you create them.