Black-and-white PDF export regression in Pillow 8.3.x

[za3k]

Black-and-white image export to PDF seems to blanket fail starting in PIL/Pillow 8.3.x. The version was reported to me by a user--I have not verified the switchover point.

Below is code which causes PDF generation to succeed (print a rectangle) in greyscale, but fails for black-and-white.

#!/usr/bin/env python3
import PIL, PIL.Image, PIL.ImageDraw

# Works in 8.2.0, fails in 8.3.0
IMAGE_MODE="1"
WHITE=1
BLACK=0

# Works in 8.2.0 and 8.3.0
#IMAGE_MODE="L"
#WHITE=255
#BLACK=0

page = PIL.Image.new(mode=IMAGE_MODE, size=(200, 200), color=WHITE)
page_draw = PIL.ImageDraw.Draw(page)
page_draw.rectangle(((0,0), (99, 99)), outline=BLACK, fill=WHITE, width=1) # Rectangle for debugging print cutoff, and to look nice.

with open("test.pdf", "wb") as f:
    page.save(f, format="pdf")

Reproduced using two different Linux versions in Python 3.x, and multiple Pillow versions.
This bug report was verified with Python 3.10.2 and Pillow 9.0.1

Originally reported as: za3k/qr-backup#24

[za3k]

fail.pdf
success.pdf

Both files display fine on Chrome web preview, but on almost every other PDF viewer tried, one looks like random garbage.

[radarhere]

While macOS Preview also displays the PDF correctly, I can see that Firefox does not.

Testing, I found this is due to #5430

[za3k]

From the original reporter: Moon Reader on Android; Firefox, Libreoffice Draw, and Evince on Linux

From me: xpdf on Linux

Aside: Looks like a great feature! I love tiny PDFs

[radarhere]

I've created PR #6151 to resolve this.