Dec 08

I wanted to print a large Christmas banner and the printer wanted an image file (png or jpg). Our file was in a pdf format. I needed to convert the pdf file to a very high resolution png file so that it would look good on a large banner. Here’s how to do it using Apple’s built in Preview app.

Here’s a picture of the file I had. It was a postcard designed by a graphic designer for our Christmas meetings. I wanted to print it onto a banner. If it was a PNG file I would not be able to blow it up but because it is a PDF file with the font in built it means I can make it as large as I can and it still looks good. But our printing shop wanted it as an image file not a PDF.

Step 1. Open the pdf file with the free ‘Preview’ app built in to Mac OS.

Step 2. Select Export from the file menu.

Step 3. Select a very high resolution. The lowest resolution you want for printing is 300 dpi. 600 dpi is good. 2400dpi is very high quality. Here I have selected 3000 dpi. dpi stands for dots per inch. The file will not really be 3000 dpi, because I am blowing it up. If you take a 300 dpi image and make it 10 times larger then the output will be 30 dpi and look very dotty. So the original file needs to be 3000 dpi for the final one to be 300 dpi if you want to make it 10 times bigger. It’s a bit of trial and error but you can export the file and see what the exported file looks like then go back and do the process again with a larger or smaller number.

Click on the file you ave exported and then select ‘Get info’ from the File menu. Here you can see the file size and the final output size.

Here you can see that the file size is 8.5 MB. That’s quite reasonable for a large image and it is still small enough to email.

You can also see that the dimensions are roughly 12,000 x 17,000 pixels.

This should blow up OK to an A0 size or large banner.

Size Guide

As a rough guide (for 300 dpi print – the minimum)

Here’s how to work it out.

An A1 page is 594 x 841mm or 23.4 x 33.1 inches.

To go from mm to inches you divide by 25.4

So to get DPI you can multiply the inches by 300.

e.g. 23.4 x 300 = 7020

or multiple the mm by 300 but divide by 25.4 as well.

e.g. 594 / 25.4 * 300 = 7016

Using this we come up with the following sizes for printing a poster.

A1 = 7016 x 9933 pixels

A2 = 4960 x 7016 pixels

A3= 3508 x 4961 pixels

This will give 300 dpi but anything smaller than A3 and I’d aim for a higher resolution. (This is because your eyes are expecting better quality on smaller prints.)

By the time you get down to an A4 page I would be trying to print at least 600 dpi minimum, so again this would be 4960 X 7016 pixels.

Applying this to a banner, I you want a banner 2m high..

2000/25.4*300 = 23622

If you wanted a 2m tall banner in good quality that’s the number of pixels you’d need in the image file for the height of the banner. Mine is 17,000 so that’s almost there.

My file

When I changed by output resolution to 5000dpi it gave a dimension of 29323 pixels and the file size was 17.5MB. This is too large to email, but if I take it on a thumb drive to the printer it will give a nice output. If it was an indoor banner that people were viewing close up I may go to this trouble. But if it was a banner outside that people were viewing from a distance I think the 17,000 pixels will be fine.

2 Responses to “How to make a very high resolution file from a pdf.”

  1. My macOS 10.15.7 “Catalina” uses Preview Version 11… and although I can Export a PNG to another file format, it does not show a “Resolution” field, where I can specify the output size.

    Can you tell me how to make mine like yours? Thanks.

    • Wayne says:

      If the file is already a png file you cannot increase the resolution. Well you can, but it won’t make it any better looking
      This only works with a pdf file because the pdf file storesthe actual information about where the lines go and which fonts are used etc. When you export from a pdf to a png it creates the image file right then and there in the resolution you specify. (This is called rendering).
      Once you have a png or jpg all the information about the fonts and lines etc is lost and the image is just a mass of dots of different colours. So you cannot increase the resolution of an image file once it has been rendered.
      You can zoom in to a pdf file and the edges the lines remain smooth and you will never see dots. If you zoom in on an image file you will start to see dots at some point.

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