Why Merged Cells Break PDF Table Extraction and How to Work Around It

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PDF files store table data as raw positioned text spans rather than structured rows and cells, making extraction inherently error-prone. Merged cells worsen this by causing popular libraries like pdfplumber and Camelot to misattribute text or generate phantom columns. The problem intensifies in multi-column page layouts, such as regulatory filings from DART, EDINET, and HKEX, where table coordinates from adjacent columns overlap. A two-stage approach — tracing explicit line segments first, then falling back to page-level whitespace gaps — can reduce extraction errors in bordered and borderless tables alike. For merged headers, a propagation heuristic that tags sub-columns as inherited from the spanning cell helps preserve correct attribution during downstream data processing.

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