It sounds like the PDF isn't being made. I would run through the Sage PDF Converter troubleshooting doc, but to rule out network (or even printer) permission issues, I'd try logging into that PC as admin and testing creation of a PDF.

I'd be more inclined to suspect a problem with the PDF Reader / file associations (...because I've seen registers want to open in a web browser). If Adobe updated, but something went wrong with the install (not done as admin) then that could break a workstations ability to open any PDF file, which has nothing to do with Sage 100 or Paperless Office.

I would be inclined to have the same initial suspicions as Kevin M, you may want to reinstall adobe reader and make sure it is the default for PDF files.

I tried a LOT of things to get this to work. It's a Windows 10/Adobe conflict with numerous suggestions all over the web.
In the end I gave up and selected the EDGE browser as the default program to open PDF files - windows suggestion.
So it doesn't work but I have a work around for now.

Sorry to hear that you could not get the issue fixed, but it is good that you found a work around.
Although Windows 10 is currently only a supported workstation platform for the Sage 100 /100c 2016 version (see Knowledgebase Article 67151 at https://support.na.sage.com), others are able to create PDFs in Paperless Office but most commonly you may receive a "unable to process lockfile" error message (see Knowledgebase Article 68571).
[A] The solution to that issue in may be helpful:
Configure Sage 100 ERP to Run as Administrator
1.Right-click the Sage 100 ERP shortcut and select PROPERTIES
2.Select Compatibility tab
3.Select the Run this program as an administrator checkbox ◦Note: If multiple users use this workstation select Change Setting for all users and check the Run this program as an administrator checkbox
4.Click OK to save changes
Or:
Set permissions for the following keys in the Registry (at the Workstation level):
1.Click the Start button, click Run, and enter Regedit
2.Navigate to the Hotkey Current Config / Software Key.
3.Right-click Software key, and then click Permissions.
4.Select Full Control for Users listed in this box, and then click Apply.
Note: UAC (User Account Control) must be On at least Level One in order for this to work. If a Program is run "As Administrator" on Windows 8+ with UAC off, functionality will be disabled.

[B] Some additional questions if you would like to respond:
1. Did the OS on the workstations change during the time that it stopped working?
2. Are the PDF files actually created but do not preview?
a. Are you able to manually browse to the folders where the PDFs are stored and view the PDFs on that same workstation by double-clicking the PDF files?
b. Did you try uninstalling the Adobe Acrobat Reader currently installed (as suggested by others) and then installing Adobe Reader X (10.1.4) from our Sage 100 2015 installation files (see KB 62994) ?

OS did not change but had changed some weeks prior.
PDF files are created but do not preview, nor can they be viewed in Paperless Office.
Adobe doesn't open another PDF files on the machine for that matter.
If fact Adobe Reader doesn't run at all.
Uninstalled and reinstalled Adobe Reader as administrator - nothing.

I thint it's a conflict between Adobe Reader and Windows 10.
Some update of one or the other caused it.
Hopefully it will resolve itself the same way.