Fix: Implement add() method in BaseFileKnowledgeSource to solve PDFKnowledgeSource instantiation error

Co-Authored-By: Joe Moura <joao@crewai.com>

tests/knowledge/test_knowledge_source_instantiation.py

@@ -0,0 +1,16 @@
+from pathlib import Path
+import pytest
+from unittest.mock import patch
+from crewai.knowledge.source.pdf_knowledge_source import PDFKnowledgeSource
+
+@patch('crewai.knowledge.source.base_file_knowledge_source.BaseFileKnowledgeSource.validate_content')
+@patch('crewai.knowledge.source.pdf_knowledge_source.PDFKnowledgeSource.load_content')
+def test_pdf_knowledge_source_instantiation(mock_load_content, mock_validate_content, tmp_path):
+    """Test that PDFKnowledgeSource can be instantiated without errors."""
+    mock_load_content.return_value = {}
+    
+    pdf_path = tmp_path / "test.pdf"
+    pdf_path.touch()  # Create the file
+    
+    pdf_source = PDFKnowledgeSource(file_paths=[pdf_path])
+    assert isinstance(pdf_source, PDFKnowledgeSource)