Basically, I have a physical catalog of wallpapers. Took pictures of everyone of them and want to convert it into pdf to share it over whatsapp. Each catalogue has about 80 images and each one is about 25mb in size. How can i compress it down so that i can fit about 70 or 80 images into a single pdf? Or is there some website i can use where can i put my catalog of images into a book form and then share the link on whatsapp?
Get a layout design program such as InDesign (high cost, but popular) or Affinity Publisher (low cost, tries to be an alternative for InDesign) or Scribus (freeware)
Make your design. The design file has only links to the image files, it can well handle your images.
Export or print as PDF. You can select the actual resolution and compression of the photos in the resulted PDF. At least InDesign has perfect export presets for PDFs which are intended to be watched on the screen.
The mentioned programs are complex because they must cover complex layouts with detailed typesetting and different color profiles. Prepare to spend a day with tutorials before you get anything useful even for onscreen watching.
Learning to make impressive layouts is another problem. Knowing program tricks isn't the same thing as artistic ability and experience in creative work.
There's also simpler programs which can do the job such as MS Word, Publisher or Powerpoint or their freeware counterparts - Open Office, for example. They cannot be used for detailed print layout work due limited typography capabilities and other limitations in color and layout possibilities, but they can be enough for your purposes. Try them!
Finally: There's several online photo collage making websites. You select the photos, the website makes the collage (and maybe gets your photo- read the terms!). A little different thing is that commercial photo book printers offer their layout design programs for making printed books. Some of them output also PDFs. I guess (never used one) you must pay for it as much as for printed book.
A very simple and completely free method would be to use a word processor such as LibreOffice Writer. Once you've created your document, export as PDF. Choose JPEG compression, set the Quality to 70% (the lower the number the more compression will be added), and reduce the resolution. Something like 150dpi should be fine for on-screen viewing, in fact you may get away with even less if required.