I am using SharePoint Server 2013 (February 2015 CU installed) to index a couple of document libraries on the local SharePoint server.

For some small Word documents (docx, less then 400 kB, less than 30.000 words and less than 2.000.000 characters) I get the following error message:

This item was partially parsed. The item has been truncated in the index because it exceeds the maximum size. ( Document 'http://demo2/....docx' was partially processed. The parser was not able to parse the entire document.; ; SearchID = 178285BA-3D51-4054-8C4F-6FBA54D078A2 )

Digging in the ULS log I found this entry:

DocParser - A parsing failure caused the partial processing of the document. The original error code was 309003 and the error message was...

The only thing that these documents have in common is that they have embedded contents such as pictures or small text files.

The logs (ULS, set to verbose for everything) give me absolutely no clue why the document was truncated. I could not find any reference to the error code. How can I find out what the issue with these documents is?