How to detect with how much dpi a pdf doc was originally pri

[mattad]

When I print a pdf from an application (browser/Word processor) then I can select the
dpi value for printing (mostly 300 or 600 dpi).

Ok, now lets say I got an already printed pdf doc.
How can I detect with how much dpi this pdf doc was originally printed?

Matt

[Stefan - PDF-XChange]

Hello Matt,

Most of those printing applications are designed to print on a physical printer, and there the DPI does have meaning.
When creating a PDF though, the situation is a bit different - a PDF document might contain data that can be reproduced as vectors - e.g. Text, annotations like lines, circles etc, for which the DPI is meaningless - vectors can be rendered and printed with any resolution you like.

The only objects that would be affected by the DPI when they would be printed are the images in such a document (if any).
Images in a PDF document are also stored in a specific way - that allows the document to scale them, so you might have a 3600x3000 pixels image, that on a 300 DPI printer would print as 12x10in , but such a high res image might be scaled in the PDF file and measure just 6x5 inches - giving that image an effective resolution of (up to) 600 DPI.

I hope the above explanation answers more questions than it creates and if you are concerned about some image's quality when printed - you could always extract it with our PDF Tools - measure it's dimensions in pixels, then measure it's "physical" dimensions inside the PDF page using our measuring tools, and try to calculate it's effective DPI inside this document.

Regards,
Stefan

[mattad]

Ok, thank you.

that means: Even if I print a web page/word doc with lets say 150 dpi the text in the resulting pdf can be printed later with 600 dpi on a physical laser printer.

Is this correct?

[Stefan - PDF-XChange]

Hi mattad,

Yes that is correct - text will not be affected, but if that webpage also has images in it I am afraid that the quality would be as good as it is on the page - usually 72/96DPI, or lower if you decide to print with lower res.

Regards,
Stefan

[Lzcat - Tracker Supp]

Hi mattad.

Actually DPI is always important, even when printing text to PDF. In most cases text quality does not depend on the DPI (except in cases where visual effects are used and the program will rasterize the affected part), but the letters position depends on the DPI, so in most cases a higher DPI is better (even the resulting file may become a bit smaller when using our print driver). You may try to print the same text with 600 and 150 DPI (for example) and see difference. It is not important if the font size is big, but for small fonts even one pixel may result in a significant position shift. In the same way DPI affects non-raster drawings (higher DPI - better quality).

The disadvantage of high DPI is that some programs sometimes scale images before printing and this will produce larger files, but in most cases programs pass the original image to the printer and ask it to scale - in this case the resulting file size will be not affected.

It is recommended to test how increasing DPI affects the resulting files and processing speed then select optimal DPI. In "typical" cases 150 DPI is not enough, 300 is acceptable, and optimal results are produced when using 600 or 1200 DPI (depends on file).

And for sure you will be able to print the PDF file to a physical printer with any DPI regardless of what is used on file creation.
HTH.