Numerous applications are available when it comes to the portable document format or PDF. Some of these applications are converters, meaning they can transform files in a different format to PDF and vice versa, while others are viewers which means they open and read PDF files. For the purpose of dividing a big file into smaller files, you're going to need a creator, a software that can make and edit PDF files.
Use a PDF creator.
Install the PDF creator, launch it and open the appropriate document. Perhaps the most popular PDF creator application is Adobe Acrobat (not to be confused with Adobe Reader which is just a viewer). Acrobat however is proprietary software. If you want to save a few bucks you can search the Internet and look for free or open-source equivalents.
Extract the pages that will compose the smaller files.
PDF creators naturally have editing tools. One such tool is to extract pages from the document and export these as a separate file. The process described here is from Adobe Acrobat's interface but other PDF creator software more or less follow the same procedure. Click on Documents on the menu bar and from the drop down list choose 'Extract Pages'. On the Extract Pages dialog box that will appear, enter the range of pages you want to separate from the original document.
Choose how you want the extraction to happen.
There are two options on the Extract Pages dialog box just below the input field where you enter the range of pages. One is "Delete Pages After Extracting" and the other is "Extract Pages As Separate Files". Putting a check on the first option will remove the extracted range of pages from the original document. Checking the second option meanwhile will create one-page PDF files for each extracted page in the inputted range. To create a single PDF document composed of all the selected pages and leave the original document intact, don't select or put a check on any of these two options. Decide on how you want to go about breaking up the PDF file into smaller parts and check (or don't check) on the appropriate options. Click on the OK button after you've made your decision.
Choose where you want the extracted smaller file to be saved.
After you click on the OK button, the next dialog box to appear will ask you to select a folder or location where the extracted smaller file will be saved. You can choose a pre-existing folder but the dialog box also offers the option to make a new one. The extracted file is given a default name in the format "Pages From (filename of original document) + (range of pages)". Click on the OK button to save the new file. You can always change the filename into something more appropriate or readable later on.
To completely break up the original PDF file into smaller parts, simply continue repeating the page extraction process until all the original pages have been taken out and saved as separate files.