During the installation of PDF2XL, your system may have provided you with the opportunity to make PDF2XL your default application for all PDF files. If you've accepted that change, then clicking on any PDF file will open PDF2XL.
You can quickly change this option.
1) Right-click on any PDF file and select "Properties" from the menu.
2) Click "Change" beside "Opens with"
3) Select Adobe Acrobat (or whichever other application you want to be the default for your PDF files) and click "OK".
Once done, your PDF files will open with your preferred default application.