Slice tall image into a multipage PDF

I have a 1200×7200 pixels PNG, and I want to slice it into a multipage PDF, 8.5×11 inches, with 1 inch margins.

For convenience, here is an inches-to-pixels table:

Inches	Pixels, at 72 PPI	Pixels, at 300 PPI
8.5	612	2550
11	792	3300

I tried

# version 1
magick input.png -crop x3300 +repage -units PixelsPerInch -density 300 output.pdf

but it doesn't leave margins.

Using trial and error, I came up to

# version 2
magick input.png -crop x2700 -repage 2550x3300+100+100 -units PixelsPerInch -density 300 output.pdf

but I cannot figure out why the text on the last page is moved to its bottom (to fix this, I tried to add -gravity center, but this removes margins).

I also don't understand why, to create margins, the value 100 works for me. Because I use -units PixelsPerInch and -density 300, I expect that 1 inch is 300 pixels and not 100, but changing -repage 2550x3300+100+100 to -repage 2550x3300+300+300 makes the margins extremely large.

The image is uploaded here: https://github.com/jsx97/test/blob/main/input.png

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And here is a visual representation of what I mean to achieve:

The opaque gray rectangles represent the sliced images. The red arrows represent 1-inch margins.

The image should extend from the top-left 1 inch margin to the bottom-left 1 inch margin. Except for the last page, of course, because the remaining height of the original image can be smaller than 9 inches.

Which version will be used, A or B, is not what the user decides. It simply depends on the image width and height.

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I do understand text and images can be sliced at arbitrary height, and this is not a problem.

magick input.png [+repage] -resize 1950x\> -crop x2700 +repage -background white -extent 1950x2700 -bordercolor white -border 300x300 -units PixelsPerInch -density 300 output.pdf

The related discussion is on GitHub:

https://github.com/ImageMagick/ImageMagick/discussions/7402

+repage after the input file is optional.