Articles here about PDF and its problems

PDF, Live Text and Spotlight: troubled relationships
How to compare two PDF documents
1 At the heart of macOS
2 Why PDF is so odd
3 Building your own PDF reader 1
4 Building your own PDF reader 2
5 Building your own PDF reader 3
6 Quartz and Preview
7 Adding a PDF Help book
8 Security, integrity and forensics
9 Bizarre bug in macOS
10 PDF Expert
11 PDFpenPro 10
Synchronised Scrolling: a brief movie on PDF
Searching PDF documents: a new feature in Podofyllin
12 Vulnerabilities in digital signing
Recovering a damaged document
13 PDF documents can readily leak data
14 PDF is inherently not secure
15 macOS can sanitize your PDFs
16 Reading PDF source
17 Unredacting Manafort’s documents and recovering PDF versions
18 Seeing inside PDF more clearly
Further improvements to Podofyllin’s Source window
19: Seeing inside PDF with Podofyllin
DelightEd now opens the rich text content of PDFs, as well as HTML, RTF and text files
Printing without tears in Dark Mode (and exporting to PDF)
Inside the Mueller Report with Podofyllin: can it be deredacted?
20: Outlines, Bookmarks and Preview
21 PDF Annotations
22 The fractious PDF
23 The nightmare of forms
28 years after Unicode, we still can’t handle accents: PDF + macOS + URL = chaos
24: Accessibility with PDF/UA
25: If at first you don’t succeed, try OCRing again
26: A PDF protection racket

Redact and obscure: how to release or publish without leaks
Redacting a PDF using Adobe Acrobat
Problems of PDF (17 February 2019)
How Preview mangles annotations
How effective and safe is Preview’s redaction tool?