Beginner needs an explanation about pdf size

[JuDrus]

Hello,

I created a mini-portfolio with my project examples and I got a 9.72mb pdf...

Then I wanted to reduce this size, so I reduced my images in that file from 25.6mb to 11.5mb...

And then magic happened... I exported my new document with reduced size images and got a 10mb pdf...

I reduced images by 14.1mb, but got a bigger pdf... How? Why?

[rob day]

Unless you have turned off downsampling and compression in the Export Compression tab, reducing the size of placed images would have no affect on the PDF size. Use the Compression tab in the Export dialog, which will allow you to sample down and compress all of the image links in the document. This would downsample all images with an effective resolution over 100ppi, and apply a Medium JPEG compression to each.

[JuDrus]

Reducing the size of placed images would have no effect on the PDF size? If I would use 10mb picture and 1mb picture, the size of PDF would be the same?

Also, my Compression tab in the Export dialog don't have those settings like yours  

[BobLevine]

That's the interactive PDF export. You need to choose the Adobe PDF (Print) export.

[JuDrus]

But mine PDF is interactive and won't be printed.

[Test Screen Name]

It depends what you to do "reduce image size". A PDF isn't a bucket containing copies of your image files. The pixels of the image are read and recompressed.

So, for example, reducing the number of pixels in the images should make a difference. While using harsher JPEG compression on your image will leave a file the same size, but ugly. So please tell us how exactlu you reduced your images.

[JuDrus]

I reduced number of pixels to 1000x1000 for every image, because some were 6000+ and some 2000+

[rob day]

With an Interactive PDF export you would have to consider the Effective Resolution of the placed images, and not their pixel dimensions. See Effective resolution in the Links panel

If you were to place the 6000px x 6000px images, and scale them to 3" x 3" on the page, their Effective resolution would be 2000ppi (6000/3). With an export to Interactive, you have to choose a downsample resolution as pixels per inch in the Compression tab (with Print PDFs you can choose no downsampling). So if you choose 300ppi all of the images will get sampled from 2000ppi to 300ppi on export.

If you were to reduce the linked image’s resolutions to 1000px x 1000px, and kept the scale as 3", the Effective Resolution would now be 333ppi, and an export to interactive with the Resolution set to 300ppi would produce the same result—the 333ppi images would also get sampled down to the same 300ppi resolution.

[JuDrus]

I really don't want to reduce below 300ppi because you can see how quality drops and everything becomes out of focus / blurry 😞

For example, this is 300ppi

and this is 144ppi...

I think that the quality of 144ppi is terrible already, so I didn't even tried 96 or 72...

Also, this is info about that image.

Why it's scale 15.9% ?

And why exported 144ppi looks much worse than the original file of 72ppi?

[rob day]

Why it's scale 15.9% ?

Because you placed the file, and scaled it down on the page. A 1000px x1000px image at 72ppi has an output dimension of  13.8889 x 13.889 inches (1000/72=13.8889):

Scaling the image to 15.84% scales both the output dimensions (13.889 down to 2.2") and the output pixel size (1/72" down to 1/455")

And why exported 144ppi looks much worse than the original file of 72ppi?

Your export of the 455ppi image to 144ppi resamples the original 1000px x 1000px image to 317px x 317px. You can see an example of that export resample in Photoshop’s Image Size dialog:

[JuDrus · Accepted solution]

Thank you @rob_day for your incredible knowledge and detailed explanations with pictures! I learned so many new things that would help me in the future!

After playing with all the settings, I found a solution that worked for me and my original 10mb file, went to 2.31mb.

The solution was kinda simple. While exporting, in the Compression tab I selected an Automatic compression method instead of default JPEG 2000 (Lossless)... and JPEG Quality from Maximum, I set it to High.

Now you can barely see the difference! And the size became 4 times smaller!

JPEG 2000 (Lossless) - Maximum - 300

Automatic - High - 300

[Test Screen Name]

Oh, and indeed, if you just scaled it down in InDesign that's truly going to make no difference... unless you are lucky.

[Test Screen Name]

One tool that may help is in Acrobat Pro.

Go to File > Save as Other > Optimized PDF.

DON'T SAVE. Just click the Audit space usage button.

This tells you what is filling the PDF. It's easy to assume it's the images, but better to check.

[Test Screen Name]

The "low resolution" image certainly looks poor, but I'd say the big issue there is putting text in a JPEG. Never do that! The "P" in JPEG stands for Photograph, don't use it for anything else. If it's a photograph of an object containing text this is tricky, and one of the hardest things to reproduce clearly... probably needs custom PDF creation settings, avoiding JPEG everywhere.

[Gusgsm]

@julius_dru

Export to Interactive PDF is fine for a portfolio to be seen only on screens. I'd try generating the PDF with 96 ppp, JPEG compression (maximum) and I'd go for 144 (as much) only if 96 caused annoying artifacts in the display of the pictures. Take note that JPEG compression and JPEG 2000 are no the same, specially if you are interested in losing weight (yes, pun intended).

Besides, it's not the same making a so-so-size PDF and then reducing it more with Acobat tools than making the PDF anew with differents settings to make it smaller. The second option will yield a much better quality PDF as you  would not be resampling the pixels twice.

But, do not believe me, please. Just give it a try. It's a very simple and fast operation.

Best regards