The PDF.MergeFromDocs operation takes a param "toAppendDocIDsVarName" to "append the blobs stored in the docs whose IDs are passed in". The value of this Context variable needs to be of type "String", but how can you specify this from an automation chain, an automation script, or via the API (including API Playground, etc.)?
A possible workaround is to use an automation script with the "PDF.MergeWithBlobs" operation, but you have to manually get the blob for the input document, and the doc and blob for each id that you pass in as a parameter. (Example available on request.)