In my experience, you get a smaller PDF if your images are placed in Indesign at or near 100%. A smaller PDF is more manageable for everyone, but I don't think that there is any compromise in output quality when letting Indesign do the scaling.
Of course, I'm only talking about reducing images - I would never let Indesign scale upwards ...
Photoshop can do a better sharping. If you need highest quality I would recalculate the images in Photoshop.
For normal purposes, you might keep them as they are.
InDesign is building up its internal preview based on the 100% size of any imported image. If you scale an image down in InDesign, its preview has therefore a higher resolution as it would have if you have scaled down the same image in Photoshop. Scaled down images are increasing the INDD file size, but have no effect on the final PDF.
The only time the PDF might be a bit smaller is if the resolution of the graphics falls between the downsample to size and the for images larger than size. In that case the images would not be downsampled.
I would never let Indesign scale upwards ...
Why not? You could easily have a case where an image is scaled up and still has an Effective resolution of 300ppi or higher.
thanks for the help!
Good point Rob, that's true - I suppose I would generally discourage scaling up because sometimes people get carried away and try to make low res images fill a page :-)
Resampling from a high effective PPI value on PDF export will tend to leave images a little soft due to the resampling process.
>700ppi effective resolution resampled to 300ppi at final size during PDF export:
300ppi resolution at final size sharpened in Photoshop:
If there are only limited images, they can be selected in Acrobat Pro one by one and then opened into Photoshop, sharpened and then saved and updated back into the PDF.
A more productive method for many images or PDF files would be to use Enfocus PitStop Pro or PitStop Server, which can sharpen images in the PDF.
There are numerous scripts available that will duplicate the InDesign file and links, resample the images to target effective PPI at the final size and relink etc. One can then run a batch sharpen and update the links, then produce a PDF that would include the sharpened files.
Another issue that can sometimes rear it’s ugly head is aliasing caused by the resampling process, which can be avoided by “intelligent” (a human looking out for it) resizing: