In Assets app, users are unable to search for doc, docx, or pdf file content.

The query created (from SearchJcrContainer) is (for example):
select * from [nt:base] as t where (([jcr:primaryType] = 'mgnl:asset' or [jcr:primaryType] = 'mgnl:folder') and (lower(localname()) LIKE 'is a test%' or t.['is a test'] IS NOT NULL or contains(t.*, 'is a test')) )

but in JCR the file content is stored under mgnl:resource node type (see example attached). The test document used is also attached.

You are able to search for the file content by adding the mgnl:resource node type to the search query (example below, you can test in JCR Tools app).

select * from [nt:base] as t where (([jcr:primaryType] = 'mgnl:asset' or [jcr:primaryType] = 'mgnl:folder' or [jcr:primaryType] = 'mgnl:resource') and (lower(localname()) LIKE 'is a test%' or t.['is a test'] IS NOT NULL or contains(t.*, 'is a test')) )

An appropriate solution still needs to be determined and this ticket will need to be updated.