An ordinary PDF keeps its index at the end, and as qpdf’s manual puts it, “the entire file must be present before it can be rendered”. Linearization moves what the first page needs, and a first index, to the front. The standard has allowed it since PDF 1.2. Acrobat calls it fast web view: its PDF Optimizer option “Restructures a PDF document for page-at-a-time downloading (byte-serving) from web servers.”

What is inside a linearized PDF?

Objects in a set order, plus hint tables “that enable efficient navigation within the document”. The first object announces the layout. qpdf and Poppler’s pdfinfo show it on any PDF; a stock Mac has neither, and brew install qpdf poppler adds both. qpdf --linearize "Demo Letter.pdf" letter-linearized.pdf turns the demo letter made for PDF Textfix’s screenshots into one, and its first object reads:

3 0 obj
<< /Linearized 1 /L 45464 /H [ 743 137 ] /O 6 /E 44525 /N 1 /T 45286 >>
endobj

/L is the file’s exact length, /H locates the hint table, /O is the first page’s object and /N the page count. The copy is 405 bytes larger than the original.

How do you check whether a PDF is linearized?

$ pdfinfo "Demo Letter.pdf" | grep Optimized
Optimized:       no
$ pdfinfo letter-linearized.pdf | grep Optimized
Optimized:       yes
$ qpdf --check-linearization letter-linearized.pdf
letter-linearized.pdf: no linearization errors

Poppler’s pdfinfo calls it “Optimized”; qpdf also checks the hint tables.

Is a linearized PDF the same as PDF/A?

No. Linearization is about the order of the bytes, for downloading. PDF/A is an archival standard about what a file may contain. A file can be either, both or neither.

Does editing a PDF keep it linearized?

Usually not. An appended save changes the length that /L records, and the standard says the result “is no longer linearized and subsequently shall be treated as ordinary PDF.” A full rewrite is linearized only if the writer asks for it.

PDF Textfix’s Save writes the whole file again through qpdf with linearization off, so a linearized file saved with an edit comes out ordinary; saving with no edit writes it back byte for byte. To restore it, run qpdf --linearize edited.pdf out.pdf.

When does linearization matter?

When a large PDF on a website opens in a browser that fetches it in pieces. A viewer that ignores it loses only speed, and a file opened from your Mac’s disk or an email arrives whole, so there it changes nothing you can see.

Questions about linearized PDFs

How do I turn off fast web view?

Write the file again: qpdf in.pdf out.pdf linearizes only when asked with --linearize.

Does linearization make a PDF smaller?

No. It adds a hint table and a second index, 405 bytes on the demo letter.

Does a linearized file display differently?

No. The pages are the same; the file only reorders its objects and adds the hint table.