Technically not all of these are malformed (sometimes the document is well-formed ISO PDF but the software won't accept it), but this corpora has a dump of all

Technically not all of these are malformed (sometimes the document is well-formed ISO PDF but the software won't accept it), but this corpora has a dump of all PDFs that were reported problematic in many software including Ghostscript, PDF.js (Mozilla) and PDFium (Chromium): https://www.pdfa.org/a-new-stressful-pdf-corpus/ https://www.pdfa.org/a-new-stressful-pdf-corpus/ (note that the majority of them are relatively-harmless rendering issues but some PDFs here have caused crashes or even RCEs and process takeovers for certain malicious PDFs)