I have an ID document with pictures and texts that I need to resize down and save as a JPEG or PNG for a webinar.
I don't know how to do this, please help me.
If it's a long document, in InDesign select File > Adjust Layout and set the new page dimensions, in the following dialogue box, ticking (probably) Adjust Font Size and possibly Auto Adjust Margins Layout (you need to experiment so do it on a copy of your original document).
Then from the new, resized document File > Export and select JPG or PNG from the drop-down Format menu and select the number of pages and resolution you want and Export it to where you want the JPGs or PNGs saved.
If you export to a .jpg format and set a low resolution this means a resized down image. Maybe this is enought for your purpose. Try it first as it is the fastest solution.