ISkysoft PDF Editor does not like longer file names or possibly accessing server stored files for combining PDF's.

Workaround:

1. Create a folder on the c: drive i.e. "working combined pdfs"

2. Right click on folder and select "Pin to Quick Access"

3. Select files to be combined and use "CTRL" and "c" to copy files

4. Open working folder created on c: drive and use "CTRL" and "v" to paste files into folder

5. Open iSkysoft PDF Editor and select "combine pdfs"

6. Go to "Quick Access" when selecting files and open and select fines in working folder created earlier

7. Complete process