Well there can be many ways to do so. Since you haven't mentioned anything clearly about what sort of the PDF file do you have. but still I would like to answer in the best possible way. One of the reason of large size of the PDF file could be the sizes of the images inside the PDF file(Assuming that the PDF has images in the file). Convert the PDF file to DOC format. Then you need to have the the pictures extracted out of that converted DOC file. Change the format of these pictures to JPG or some other such formats that don't take too much of space. Put the pictures back into the DOC file. Convert it back to the PDF and its size will be reduced. There are other ways through some other softwares to just compress the PDF file and t ask you about how much size would you want it, at the time of conversion, but t does not compress it as much as manual methods can reduce the file size.
However, I think I can answer both questions at the same time. I don't have time to check in to the answer, but this is my best guess. Just like if you have a large collection of images inside your PDF file, but you only want 1 or 2 of them, there is a way to remove those in a very quick way. There is a way to save a copy of the original images of the PDF file to your computer, so when the PDF file comes back later, you can easily restore the one that you removed from the PDF. Convert the PDF file back to DOC format. Now put the extracted files back into the DOC file. I would go and find a program that does extract files and put them in a folder. Now with this program, you can make the pictures as the size as you want, which could.