Transparent PNG to PDF: what happens to the background

The short answer: see-through becomes white, the rest is kept

When you turn a PNG with a see-through background into a PDF with PNG to PDF, the background becomes white, like paper. Everything you can actually see stays: colours, thin lines, the edges of letters. Here is the whole story in four lines:

• Fully see-through parts turn plain white.
• Partly see-through parts, such as soft edges and shadows, mix with white and look paler, as they would on a white website.
• Solid parts are kept pixel for pixel, with no loss.
• White or very light parts on a see-through background seem to vanish, because they are now white on white.

In our tests in Chrome on a computer, a signature on a see-through background came out as a clean blue signature on white, and a colour logo looked just as it does on a white website. The white lettering of our own white logo disappeared completely, which is the one thing to watch for.

What it can't do is keep the background see-through inside the PDF. If you need that, see what to do instead.

Which kind of PNG have you got?

Not every PNG is handled the same way, and you can't always tell the kind by looking. This is what the page does with each:

The tricky part: a PNG can carry a see-through layer even when every pixel in it is solid, as some apps and phones save screenshots. It looks the same afterwards, but it is stored again rather than copied. The result line tells you, in one of these sentences:

• "Every picture went in exactly as it was, at full quality." All your PNGs were plain. With one picture, it says "Your picture went in exactly as it was".
• "Your picture was stored again without any loss." It had a see-through layer, or was interlaced. Every pixel you can see is kept, and see-through parts are white.
• "2 pictures went in exactly as they were; 1 was stored again without any loss." A mix. The counts change with your pictures.

Fully and partly see-through: what you will see

Every pixel in a see-through PNG has an amount of see-through, from fully solid to fully clear. The page lays the picture on white in your browser, so each pixel is mixed with white by that amount. We tested a signature and a logo, not every kind of shadow or glow, but this is how the mixing works:

• Fully clear pixels become pure white. The colour hidden inside them doesn't matter, because nobody could see it.
• Half-clear pixels become a halfway mix of the picture's colour and white. A dark blue at half strength comes out as a lighter blue.
• Soft edges are really a thin ring of partly clear pixels. They are what makes a round logo look smooth, and on white they still do.
• Shadows and glows are partly clear all through. On white, a grey shadow shows as a paler grey, and a white or very light glow almost disappears.

In short, you get the picture as it would look if it were printed on white paper. For most logos and signatures, that is exactly what you wanted.

One small change in how it is stored. A picture with no colour at all after it is laid on white, such as a black signature, is stored as grayscale, which keeps the PDF small. You won't see a difference.

Common see-through pictures, one by one

If you are unsure about a picture, look at its card. The preview on each card shows the picture laid on white, the same way the PDF will have it. The guide shows what to do with white logos in three steps.

If you need the background kept, or a colour behind it

A PDF made on this page cannot be placed over a coloured design with the picture's background showing through. A PDF can hold see-through pictures, but this converter doesn't: it lays them on white, as if the picture were printed on paper. Here is what to do when white isn't what you need.

You want a colour behind the logo

Put the colour into the picture first. In any photo editor or presentation app, draw a rectangle in the colour you want, place the logo on it and export the result as a PNG. That new picture has no see-through parts left, so nothing can vanish. With Same as picture and no margin, it fills the whole page. On A4 or with a margin, there is white around it.

You want the logo kept see-through on pages of a PDF

That is a job for Add Watermark, not for this tool. It stamps a PNG, JPG, WebP or GIF picture across the pages of an existing PDF. A PNG with a see-through background works best, because only the logo shows, not a white box around it, and the see-through parts are kept in the PDF. The size is a share of the page's width. It puts the logo once in the middle of each page or repeats it, and doesn't place a picture at a spot you choose, so for that, use the program that made the document.

You want to sign a document

Turning your signature PNG into a PDF gives you a page that holds only the signature. Our tools don't do e-signing, and this converter only puts each picture on a page of its own. Put the signature on the document in the program you write it in, or ask the other side what they accept.

You want it for a website, or a design file

Keep the PNG. Websites and design programs can use its see-through background, which no PDF from this page will have. The PDF is the copy you send to people, and the PNG is your original.

You will print it on coloured paper

Most printers print white as "no ink", so on coloured or cream paper the white parts of a page show the paper.

Check the quality yourself, in four ways

"Without losing quality" is easy to say, so here is how to see it for yourself.

1. Read the result line. It tells you, picture by picture, whether it went in as it was or was stored again without loss.
2. Compare the sizes. Under the PDF's name you see the size of your pictures and the size of the PDF. For plain PNGs they should be close, often within 1 KB. Our bar chart was 12 KB and its PDF 13 KB. A typed form saved as a PNG page was 70 KB and its PDF 71 KB. A computer screenshot of 1920 × 1080 was 214 KB in both. That closeness is the sign that nothing was compressed again. Pictures stored again can differ a little, as our two see-through pictures did, because they were packed again. The signature even got smaller.
3. Zoom in on both. Open the PNG in your gallery or photo app and the PDF in a viewer, such as our PDF Viewer, and zoom into thin lines and the edges of small letters. They should match. We also had a different PDF reader (poppler) draw every page of our test PDFs, and each one matched its picture.
4. Look at the card before you convert. It shows what the page will do with the see-through parts, so you can catch white on white early.

Your original PNG is never changed, and the PDF is always a new file. So there is no harm in trying a picture and looking at the result.

Sharpness depends on the pixels, not the PDF

A PDF can keep every pixel of your PNG and still look soft. The reason is simple: sharpness is how many pixels are drawn per inch of page. The PDF keeps your pixels, but the page decides how big they are drawn.

This page always scales a picture to fit its page. With Same as picture, the long side of every page is 297 mm, the length of an A4 sheet, whatever the size of the picture. With A4 or Letter, the picture is scaled to fit the sheet. There is no option for "actual size", so a small picture is enlarged.

Here is a worked example. Say your logo PNG is 1200 pixels wide, and you put it on A4 with Direction set to Portrait, so it fills the width of an upright page, about 8.3 inches (21 cm) across. (On Auto, a wide logo gets a sideways sheet and is spread even wider.) That is about 145 pixels for each inch. That looks fine on a screen, but soft on paper, where print shops often ask for 300 pixels per inch. A logo 2400 pixels wide would give about 290. The PDF is the same quality as the PNG in both cases. The PNG itself was just small.

• Start from the biggest version you have. If a logo or chart looks blocky, export it again from the program that made it at a larger size, such as twice the width, and convert that file.
• Screenshots are fine on screens. A phone screenshot has exactly as many pixels as the phone's screen, so on a phone it looks about as sharp in the PDF as it did in your gallery.
• Zoom before you judge. A PDF opened at a small zoom can look rough just because the viewer is squeezing the page. Zoom in to see the real pixels.

What really loses quality, and how to avoid it

Converting on this page is not where quality is usually lost. These are:

• Saving the PNG as a JPG first. JPG throws away fine detail to be small, and the edges of letters and lines go fuzzy. Each time a JPG is saved again, a little more goes. Keep screenshots and drawings as PNG. A photo is the one thing that is better as JPG, as the PNG or JPG part of the guide shows.
• Taking a screenshot of the picture instead of using the file itself. A screenshot has only as many pixels as your screen.
• Getting the PNG through a chat app as a photo. Some chat apps shrink pictures sent as photos. If someone sends you a logo that way, ask for it as a document, and send your finished PDF as a document too.
• Shrinking the PDF afterwards. Compressing a PDF makes pictures lighter on purpose. Our PDF of four colour PNG scans was 11.0 MB, and our compressor brought it to 684 KB at Recommended. That saved space, but it isn't the full-quality file any more. Keep the first PDF.
• Printing a picture to PDF through a print window. That is not a straight copy of the file, and the result depends on the print settings, so run the same checks on it.
• Enlarging a small picture, as in the section above.

Some phone screenshots carry a see-through layer, so they follow the rules above. Our article on combining screenshots shows how to take them, order them and keep them small.

The fine print

• 16-bit PNGs with see-through parts are stored at 8 bits per colour, like almost every picture on a screen. You won't see a difference, but if a print shop asked for 16-bit, give them the original PNG.
• Very large pictures. Plain PNGs go in at any size. PNGs that have to be stored again are limited to 64 megapixels, or 16 megapixels on iPhone and iPad, and larger ones are scaled down.
• Text stays a picture. You can read the text of a screenshot or a scan in the PDF, but not search or copy it. The page doesn't recognise text.

Problems and quick fixes

• Part of my picture is missing. It was white or very light on a see-through background. Use a version made for light backgrounds, or put a colour behind it and export a new PNG.
• My logo has a white box in my design. The PDF has a white background by design. Use the PNG, or stamp the logo with Add Watermark.
• The result says my picture was stored again, but nothing is see-through. Normal. The PNG carried an unused see-through layer. The page looks the same.
• The PDF looks blurry. Check the sharpness section: the PNG may be small, or the viewer may be zoomed out.
• The PDF is as big as my PNGs. Plain PNGs go in untouched, so the PDF is about as big as they are. For a smaller file, compress the PDF afterwards, or use Compress PDF to 100 KB for a form with a limit.

The see-through part of the guide and the questions and answers cover more.