I have a set of white label PDF forms that I need to brand for our company. The initial document is 6 pages, 316KB, and already has a full set of fillable fields. In a perfect world, I just open up the first page, replace the header with a header that has our logo, save, and be on my merry way. If the file size jumps a bit, I can just go into Acrobat and "Reduce File Size."
However, the results were really bad. I think I about doubled the file size by doing this on one page alone. I thought the inflated size might have something to do with fonts since I had to "Find Fonts" in Illustrator, so I outlined my company's header (it used a different font) and went through every page to get all of the fonts on the same page. The results were worse: 2.49MB!
PDF file sizes have always felt like a dark art to me, and I want that to change. If my Photoshop file is huge, I know it's because of document dimensions, layers, and such. When my AI file sizes got out of control, I learned that unchecking "save PDF compatible version" would help out a lot. When I'm making my own PDFs and I want to export them, I have a solid grasp on downsampling and such to get a good file size.
But when it comes to editing PDFs, I really don't know how file sizes work. I don't know what "Reduce File Size" means. I don't know why sometimes a file size will double or triple and why it sometimes won't, and if PDFs are embedding duplicate data, I don't know how to get it out.
So, how do I keep file sizes down when editing PDFs?