I'm not familiar with the SolrSearch plugin, so maybe someone else can clarify its role here.
Still, you're likely seeing different results because the site-wide search and advanced search query different tables. The site-wide search performs a full-text (i.e. relevancy) search on the
search_texts table, while the advanced search page performs a search on the
When the PDF Text plugin detects a PDF file, it extracts the text and saves it as File metadata, not Item metadata. This is an important distinction because the advanced search only queries Item metadata, so the PDF text is invisible when using that page. The site-wide search, on the other hand, does query File metadata (because of an automatic process that appends it to the parent Item's metadata).
In short you aren't seeing PDF text in your advanced search results because the advanced search doesn't search File metadata. You are seeing PDF text in your site-wide search because the full-text table contains an aggregation of File and Item metadata.
I've created an issue on GitHub that may solve this problem, but I'm not sure if and when we can get to it.