TIFF to PDF: dpi setting of the PDF profile does not change the file size

[janvokral]

Hi,

I have a question. I'm digitizing texts (archival tiff format 400dpi), which I'm converting from color or grayscale to black and white to save space. The result is smaller tiff files, I keep only the color images or thin graphics in color. When I convert tiff to pdf and merge into one pdf file, I use the straight function in the PDF24 Toolbox Merge PDF. I assume that the files are converted in the Best Quality profile, because e.g. a 50 MB input becomes a 250-300 MB PDF.
So I decided to convert the tiff files in the second best profile Very Good Quality (in PDF24 Launcher). I see in the profile details that it should create a PDF with 600 dpi. However, after the conversion, I don't see a difference in size between the original 400dpi resolution of the tiff files and the converted PDF. Does this setting work? At the same time, if I select the Custom profile when converting. I open the Very Good Quality profile and uncheck use dpi resolution box, the resulting size is the same as when this is left checked. For example, from a 50MB input, the composite PDF is 60MB, and it is the same whether the resolution dpi is checked or not.

Can someone explain this to me? Thank you

[PDF24 Support]

Hello,

there are two different things at work here:

1. Merge PDF in the PDF24 Toolbox does not use a profile. Images are converted with the built-in image converter, whose settings are in the PDF24 Settings under Images To PDF. Black-and-white images are stored lossless with a general-purpose compression, color images as JPEG with JPG Quality 100 (the default). If your black-and-white TIFF files use fax compression (CCITT G4), as is common for archival scans, the PDF becomes much larger than the TIFF files. So the large file size does not come from the "Best quality" profile.

1. The dpi value in a profile is a maximum. Only images with a higher resolution are reduced to this value. Your scans have 400 dpi, which is below 600 dpi, so they stay unchanged, and the result is the same with or without the resolution checkbox. The smaller result (60 MB instead of 250-300 MB) comes from the profile compressing the images again, not from the resolution.

The dpi setting only makes a difference if you choose a value below 400 dpi, and then the images lose resolution. If you want to keep the full resolution, the profile you use now is the right way. For color images, you can also lower JPG Quality under Images To PDF; this makes them smaller, but JPEG is lossy.

You can also convert TIFF files online: https://tools.pdf24.org/en/tiff-to-pdf

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