Convert PNG to PDF

Why a PNG-built PDF is exactly as big as its pictures

PDF has no idea what a PNG is, but it happens to store images with the same compression PNG uses — so a PNG goes into the document without being re-encoded. Its pixel data is carried across untouched, not decoded and compressed again, which means no generation loss whatsoever and a PDF that weighs essentially the sum of the images you put in it. Eight 4 MB screenshots make a 32 MB PDF. That is not inefficiency; it is the price of a lossless format, and it is the reason a PDF of photographs should be built from JPGs instead.

Transparency is handled properly rather than flattened. A PNG's alpha channel is separated out and attached as a soft mask, which is the PDF specification's own mechanism for per-pixel transparency — so a logo with a soft shadow sits on the page without the grey rectangle that a lesser conversion leaves behind. What is behind it is the page itself, so a transparent PNG shows white unless something else was drawn there.

Page size follows what you choose. A fixed size fits each image inside the margins while keeping its proportions — never stretched, never cropped — and a portrait screenshot on a landscape page simply leaves space either side. Choosing to match the image instead makes each page exactly the picture plus its margin, so a PDF of mixed screenshots has pages of different sizes, which is perfectly legal in a PDF and occasionally surprises a printer.

One image becomes one page, in the order shown on screen, and the text inside a screenshot stays a picture of text: it cannot be selected, searched or copied. That is true of every image-to-PDF conversion anywhere. If the words need to be findable afterwards, the page has to be read — recognise the text first and keep it alongside.

When you want something else

For hundreds of images, or for anything scripted, img2pdf is the right tool and is unusually principled about it: it embeds the original data without re-encoding, exactly as here, and refuses rather than silently degrading anything it cannot embed losslessly. img2pdf *.png -o out.pdf is the whole command. ImageMagick's magick *.png out.pdf is more flexible and will happily recompress, so pass -quality 100 if that matters.

When the PDF needs to be small rather than perfect, converting the PNGs to JPG first will beat any amount of PDF compression afterwards — a screenshot-heavy document routinely drops by eighty per cent with no visible difference. The exception is line art, diagrams and text screenshots, where JPEG puts visible fringing around every sharp edge and PNG is genuinely the right choice.