For simple compression, use QPDF: it's also available in Windows, I believe there are graphical frontends too.
qpdf --linearize input.pdf output.pdf
If you want a smart compression without too much thinking and the documents are for your own usage, forget PDF and convert them to DjVu, which is much more compressed especially on documents with many pages. There are many options but pdf2djvu is the easiest:
pdf2djvu -o output.djvu input.pdf. Some GUIs available: pdf2djvugui2 (GPL), PdfToDjvuGUI (freeware).
If that's not enough, and assuming your issues are with embedded images, you don't have many options: you need to extract the images and resave them with a different quality, format or compression. It's very easy in
convert to alter JPEG quality and compression for PDF, but such rasterised operations can be quite terrible.