@online{pdfreference,
title= {PDF Reference, sixth edition},
author={{Adobe Systems Incorporated}},
url =  {https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf},
year = {2006}
      }

@book{pdfspec-iso32000-1,
  author={{Adobe Systems Incorporated}},
  title={Document management -- Portable document format -- Part 1: PDF 1.7},
  date={2008-07-01},
  edition={1},
  url={https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf},
  urldate={2021-04-18},
}


@book{pdfspec-iso32000-2_2017,
  author={{International Standard}},
  title={ISO 32000-2:2017(en)},
  subtitle={Document management — Portable document format — Part 2: PDF 2.0},
  date={2017-07},
  edition={1},
  url={https://www.iso.org/obp/ui/#iso:std:iso:32000:-2:ed-1:v1:en},
  urldate={2021-04-18},
}

@book{pdfspec-iso32000-2_2020,
  author={{International Standard}},
  title={ISO 32000-2:2020(en)},
  subtitle={Document management — Portable document format — Part 2: PDF 2.0},
  date={2020-12},
  edition={2},
  url={https://www.iso.org/obp/ui/#iso:std:iso:32000:-2:ed-2:v1:en},
  urldate={2021-04-18},
}
@online{pac3,
title = {PDF Accessibility Checker (PAC 3)},
author= {{Zugang für alle -- Schweizerische Stiftung zur behindertengerechten Technologienutzung}},
url = {http://www.access-for-all.ch/ch/pdf-werkstatt/pdf-accessibility-checker-pac.html},
urldate ={2018-07-05}}

@online{verapdf,
title = {veraPDF},
author={{veraPDF consortium}},
url = {http://verapdf.org/}
}

@online{ngpdf,
title = {Next-Generation PDF},
author={{Dual Lab}},
url = {http://ngpdf.com/}
}

@online{tugaccess,
title={PDF accessibility and PDF standards},
author= {{TeX User Group}},
url= {https://tug.org/twg/accessibility/}
}