Q: How do I make PDF Studio my default PDF viewer application in such a way that PDF files always open with PDF Studio when double-click on them?
A: Depending on your operating system, follow the steps below to change the default file association to point to PDF Studio application for PDF files:
Windows
- Right-click on a PDF file.
 - In the mouse menu, choose “Open With”.
 - Choose “Choose default program…” in the sub-menu (last entry).
 - A dialog opens.
 - Select PDF Studio in the Recommended Programs list or Click “Browse” to navigate PDF Studio (C:\Program Files\PDFStudio\pdfstudio.exe) if PDF Studio is not showing in the list.
 - Check the box “Always use the selected program to open this kind of file”
 - Click OK to close the dialog.
 
Mac:
- In Finder, select any PDF file.
 - Go to “File” and “Get Info” in the Mac OS X menu bar.
 - Expand the “Open With” section and you will see a drop-down list with the current default selected
 - Click on the drop-down list and select PDF Studio.
 - Click “Change All” and then “Continue”.
 
Linux (Ubuntu):
- Right-click on a PDF File.
 - Select “Properties”.
 - Go to the tab “Open With” and choose PDF Studio 2018.
 
If PDF Studio 8 does not show up in the list, open a Terminal window and follow the steps below.
- cd ~/.local/share/applications
 - gedit pdfstudio2018.desktop (this will open the file in gedit). Depending on the installer you used the file may be called pdfstudio2018-x.desktop with a version number.
 - Look for the line that starts with “Exec=” and add %f at the end of the line.For instance if the line reads:
Exec=/bin/sh “/home/john/pdfstudio2018/pdfstudio2018”
Change it to:
Exec=/bin/sh “/home/john/pdfstudio2018/pdfstudio2018” %f - Save the file
 - After that, PDF Studio 2018 should show as an option under “Open With” in step 3) above.
 - As a root user, open “default.list” file under “/usr/share/applications”
 - Change “application/pdf=evince.desktop” to “application/pdf=pdfstudio2018.desktop”
 - Restart computer to apply changes.
 
If the above steps do not work, try the steps in this AskUbuntu answer.