This article applies to the SP2010 workflow engine (Which is also available in newer SharePoint versions). When you experience this problem with the SP2013 (Workflow Manager) workflow engine, then see this Knowledge Base Article.
The PDF Converter for SharePoint ships with a large number of SharePoint Designer Workflow activities that make it easy to automate common PDF related tasks such as conversion, merging, splitting, securing and watermarking.
Occasionally we get reports from customers who can see the Muhimbi Workflow Activities in SharePoint Designer, but once selected they are not added to the workflow. The most common reason behind this problem is that the web.config file was not updated correctly during deployment. Perhaps the web.config file was locked or another process was accessing it.
The easiest way to troubleshoot this is to retract the PDF Converter WSP in Central Administration and redeploy it (or run uninstall.cmd / iisreset / install.cmd). Please restart SharePoint Designer as well.
If this does not solve the problem then it is worth investigating what went wrong. To do this open the Windows Application Event log and search for all entries with Event ID 41734.
If this does not shed any light on the problem, and reinstalling the software does not solve the problem either, then you can manually add an entry to the web.config file for the Web Application you are trying to add the Workflow to. Please repeat these steps on all servers involved in executing workflows (How to determine this)
<authorizedType Assembly="Muhimbi.SharePoint.DocumentConverter.PDF, Version=188.8.131.52,
Please note that <authorizedTypes> will already be present, you just need to add the '<authorizedtype' entry to the list if not already present. Line breaks have been added for cosmetic reasons, please remove all from the <authorizedtype> entry when updating your web.config file.
Muhimbi's Software uses the standard SharePoint API for deploying web.config changes. Unfortunately under some circumstances these APIs fail.