The behavior you're observing with the convert command (part of ImageMagick) is due to how the -flatten option interacts with multi-page PDFs.

Why you get a single file.png instead of file-0.png, file-1.png:

• -flatten merges all layers and pages into a single output image.
  When you use -flatten, ImageMagick combines all pages of the PDF into one flattened image, resulting in a single output file (file.png). This is the expected behavior of -flatten: it merges everything into one image, regardless of the number of input pages.

• Without -flatten, ImageMagick outputs one image per page.
  If you omit -flatten, ImageMagick will generate separate output files for each page (e.g., file-0.png, file-1.png).

How to get separate images for each page:

Remove the -flatten option:

bash
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convert file.pdf file.png

This will produce file-0.png, file-1.png, etc.

If you want to flatten each page individually:

Use a loop or specify the page range:

bash
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convert -density 300 file.pdf[0] -flatten file-0.png
convert -density 300 file.pdf[1] -flatten file-1.png

Or, in a loop:

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for i in $(seq 0 $(($(pdfinfo file.pdf | grep Pages | awk '{print $2}')-1))); do
    convert -density 300 file.pdf[$i] -flatten file-$i.png
done