Below is what user provided from pdfDocs Support in regards to this issue:

Just wanted to inform you about what we found working with PDFDocs about this problem. We were able to get the HowNow save dialogue window to open by pointing PDFDocs at HowNow5.0.0.119.exe in the HowNow directory instead of HowNow.exe. However, when we hit finish we were getting the same ‘not found’ error. From what the PDFDocs support people told me, the latest build of PDFDocs switched from deleting the temp documents when PDFDocs was closed to deleting them immediately upon being picked up by another application. They said the reason that we were having problems is that HowNow appears to use multiple executables in handling the saving of the document, and when the document was being picked up by the first, PDFDocs thought it was done and deleted the temp file. Thus, the file was no longer there when HowNow was actually ready to save it. We’ve solved the problem for the moment by rolling back PDFDocs to the previous build, 4.5.200.28, and this seems to have solved our issue for the moment.