style: add necessary comments to improve code quality

- Add docstrings and inline comments for key functions and complex logic
- Unify comment style, eliminate magic numbers and ambiguous variable names
- No functional changes, only improve maintainability

docreader/parser/pdf_parser.py

@@ -4,4 +4,13 @@ from docreader.parser.mineru_parser import MinerUParser
 
 
 class PDFParser(FirstParser):
+    """PDF Parser using chain of responsibility pattern
+    
+    Attempts to parse PDF files using multiple parser backends in order:
+    1. MinerUParser - Primary parser for PDF documents
+    2. MarkitdownParser - Fallback parser if MinerU fails
+    
+    The first successful parser result will be returned.
+    """
+    # Parser classes to try in order (chain of responsibility pattern)
     _parser_cls = (MinerUParser, MarkitdownParser)