As long as we do not know what is the content ad how you have exported the PDF, we cannot judge about the size. We don't even know the number of pages.
I have often files with 50 MB or more, I see not a problem with this size.
But what could have influence?
- Amount of images.
- Resolution of images.
- Color mode of used images, CMYK are bigger than RGB than Greyscale.
- PDF standard, as PDF/X-3 is bigger than X-1a or X-4.
- Have the images different color profiles as it would increase file size.
- Are the fonts converted to outlines (curves), which should never be done, but this would increase file size.
- Repeating elements, are they on the master or are they released or placed on the pages? On the Master they use less space if PDF is exported without flattening.
- What are your Job Options?
- Is it tagged? Even if I recommend tagged PDFs as it opens faster in Acrobat DC, it costs space.
- Ar thumbnails embedded?
Your information is scant!
Which version of InDesign and OS?
For which purpose is your PDF to be used for, e.g. printing or screen?
Are your images JPGs, how many, what are the typical dimensions, what compression. did you apply, did you resave the PDF to reduce its size further.
In Design C6 OS 10.75
Only for screen use, pdf's and e-pubs.
Yes, all the images are jpeg's.
The photos are about 9 x 7 cm's.
I do a Save as before I convert it to a pdf.
The Images are about 6cm x 8cm, 72 dpi 250 pixels. There are around 200 images in a 210 page book. the images are RGB.
I do not know what this means (PDF standard, as PDF/X-3 is bigger than X-1a or X-4) The page size is slightly longer than an A5.
The fonts are the same as I use every year.
Yes, I use the Master file for repeated data.
As far as I know I do not have any thumbnails.
I have been producing this same type of book for many years in In Design C5 and have never had this problem.
So the size of a typical image is 9 x 7cm which is 1063 x 927px, or are they 6 x 8cm (the resolution is irrelevant in this context) – what do you mean? "72dpi 250 pixels"?
What compression do you apply when you save them as a JPG?
Have you tried (within Acrobat Pro) Save > Save as Other > Reduced Size PDF?