If you're a Windows 10 user, it's likely you have your default PDF reader set to Microsoft Edge. Seeing as Microsoft Edge is app based, Simsol cannot call to it to open.
To fix this, all you'll need to do is set a different PDF reader, like Adobe, as your default program. Here's a step by step to do so:
Go to Window's Start and Open Settings
Click on Apps
Click on Default apps
Scroll down and click the 'Choose default apps by file type' link
Scroll down and find .pdf (PDF File), and click the button on the right side, which is likely to read "Microsoft Edge."
Select your app from the list to set it as the new default
Click the Switch anyway link to confirm the change
Once you completed the steps, Microsoft Edge will no longer open PDF files by default in the web browser and Simsol should be able to properly open attachments.
If you continue having issues with this feature, contact our support team by either submitting a ticket or calling us at 1(800) 447-4676.