Can't create a multiple PDF (large files)

 I use Acrobat Pro (64 bit) to create PDF files out of Tiff images. I then have to combine these PDFs into one PDF file. This work fine with small amout of PDF files but when I try the same command (combine files) on larger amount of files it comnines them into a binder but doesn't let me save the binder to my computer. I tried to reduce size of file, but also can't because if you don't save the file (and again- it doesn't let me), you're unable to reduce size. How do I work around this probelm??