PDF Sucks

The PDF format is a blight and its distribution online is making the world a worse place. The format itself is binary; you need dedicated software to read and edit PDFs. Most of that software doesn't even have advanced functionality like searching by regular expression, comparing with diff, and text extraction (which are standard in text editors). The styling and font is controlled by the document as well, instead of the reader (except in rare cases such as "document recoloring" provided by some readers, like zathura), so there's barely any customization at all, and you're doomed to read a document with a horrible, small font and a weird color contrast.

However, the main problems with PDFs are their stubborn archaisms and failed attempts to bring the quirks and limitations of physical paper into the digital world. The concept of pages, for example; digital things are scrollable, so pages are just pointless seams in the scrolling (often at egregious places such as between a heading and its first paragraph), and document scans shouldn't have that digitally. Another thing is the paper sizes PDFs can have; A4, Letter, Legal, etc. You might say these things actually do make sense for printing, but if these are the only use-cases, you wouldn't want to distribute documents in this format (as is often done); you'd only want to convert them to PDF as part of the printing process.

The other problem is that the PDF format suffers from inefficient compression algorithms and extreme levels of bloat and complexity (compared to other formats such as DJVU); have you ever tried measuring the source code of a PDF viewer? PDFs have a built-in password protection feature (which is weird, unsupported by most document viewers, and can be replaced with file encryption so there's no need for such a feature at file format level), they contain fillable forms (which share the same problems as the aforementioned), and they even contain JavaScript so you can literally run DOOM on them, and you can probably be fingerprinted and spied upon by malicious proprietary code (if you ever wondered why the CIA publicly hosts terabytes of PDF documents written by the big scary "extremist" Muslim ideologues on their website).

Many PDFs I run into could've been plain text files, or CBZ (which are just ZIP files with sorted images of comics or manga scans in them). Most of the remainder would be better off as HTML files (in fact, EPUBs are also basically just ZIP files with HTML/CSS files inside); PDFs reside on the web anyway, so their format is just pointless friction (especially if the user's browser doesn't support display of PDFs automatically).