Can't search contents of a pdf in a windows search may be caused by an iFilter not being available.
If under Indexing Options -> Advanced -> File Types, you see that the .pdf extension says "Registered IFilter is not found" then this is why pdfs cannot index with Windows Search.
Likely cause is an old version of Adobe Reader or Pro is installed. Uninstall the old product. Might also be because no Adobe product is installed, then install Reader.
Confirm under Indexing Options -> Advanced -> File Types, that the .pdf extension now says Reader Search Handler.
Rebuild the index. Note, it may resolve itself, but I just rebuild to make sure it's working on it properly.