Transparent PNG to PDF: what happens to the clear background

What transparency means on a PDF page

An image editor draws transparent areas as a grey and white checkerboard, so people expect to see one in the PDF. They never will. The checkerboard is only the editor’s way of saying “nothing here”. A PDF page is a sheet of paper, and what shows through a clear area is whatever is underneath it on the page.

Inside the file, PDF keeps a PNG’s transparency as a soft mask: a second, grey picture the same size as the first, where white means solid, black means fully clear and the greys in between are the soft edges around letters and curves (ISO 32000-1, the PDF standard, section 11.6.5). Acrobat, Preview, Chrome, Edge and Firefox all apply it.

When getPDF turns a PNG into a page, it paints the page white and places the picture on it. So:

• the clear parts of a logo show white, on screen and in print;
• a soft shadow or anti-aliased edge blends into that white smoothly, with no fringe;
• the picture inside the file still carries its soft mask, unless you flatten it.

The Transparency switch, and what each setting does

As soon as a PNG (or a WebP, AVIF or GIF) is in the list, the options show a Transparency row with the hint “Keep see-through parts (PNG, WebP, AVIF, GIF), or flatten onto white”. It reads kept by default; tap it to switch to flattened.

A WebP or AVIF with see-through parts is treated the same way: stored losslessly with its soft mask, and the same switch flattens it. Only WebP and AVIF photos with no clear parts are written as JPEG at quality 90.

Kept stores the picture losslessly with its soft mask. The page shows the logo on white. Because the clear background is still in the file, Extract images from PDF gives you back a PNG with its transparency, and anything built from the page later still knows which pixels were empty.

Flattened mixes every partly clear pixel with white, once, and stores a plain picture with no mask. The page looks exactly the same. What is gone is the information: the logo now has a white rectangle baked around it.

We measured both on a 1200 by 600 pixel stamp, a 23 KB PNG:

Kept is smaller here because a mostly empty mask compresses to almost nothing, while the flattened picture has to store all that white. Your numbers will differ, but kept rarely costs much.

When to keep it, and when to flatten

Keep it (the default) for logos, stamps and signatures you may reuse: you can take the picture back out with its clear background, and nothing is lost by keeping it.

Flatten it when the PDF goes somewhere that handles soft masks badly: an old print driver, a cutting plotter, a print shop’s checker that flags transparency, or a portal that converts uploads with an old library. Flattening removes the question entirely.

Neither, when the goal is a logo on an existing document. PNG to PDF makes a new page from each picture; it does not put a picture on top of another PDF. For that, open the document in the editor, press I for “Add a picture”, click where the top left corner goes and choose the PNG. The editor’s hint says it plainly: a photo is stored as a JPEG at quality 85, while “anything with flat colours or transparency keeps every pixel”. The document shows through the clear parts, as in the left panel above. The add an image to a PDF guide walks through placing and resizing.

The dark mode trap is in the PNG, not the PDF

A black logo with a clear background vanishes on a dark background: in a chat app in dark mode, in an email client’s dark theme, in a slide with a dark design. That is the right panel above, and it catches people who send the PNG itself.

A getPDF page does not have this problem, because the white page sits under the picture. A dark theme that recolours the whole page (some readers offer a “night” or “invert” mode) recolours logo and page together, so the contrast survives. If the logo must work on dark backgrounds as a picture, it needs a light version or an outline; converting to PDF is the other way to pin it to white.

The honest part

JPG has no transparency, so do not convert to JPG first. Saving a transparent PNG as JPG fills the clear parts with a solid colour, white in most programs, and bakes the soft edges into that colour. Put the result on a coloured background and a pale halo shows around every letter. Give the converter the PNG.

It cannot invent transparency. A logo photographed on paper, or a PNG that was saved with a white background, has no clear parts to keep. The background has to be removed first, in an image editor, before the PDF can show anything through it.

A soft mask is not a security feature. The clear areas hide nothing, and a flattened picture is still just a picture. To remove something from a page for good, use redaction, not a white box.

For which format to choose in the first place, PNG for logos, screenshots and text, JPG for photos, see JPG or PNG for a PDF; the pictures to PDF guide has the table of what each format becomes inside a PDF. To edit what is already on a page, start with edit a PDF.