I just figured this out and it's actually very simple, and once you do the first one, the rest are even easier. You can't edit the pdf or anything, you can only view it locally in Safari, so I'm not sure if this is what you want. But no internet connection is required.
Install "file:// Schema in Safari" from Cydia.
Create a webclip for a random site, let's use google for example.
I suggest using iFile (from Cydia, not the app store) for the rest of this, but there are other ways as well.
Go to /var/mobile/Library/Webclips and find the folder for the webclip you created. If you have a lot of webclips, it may take a lot of searching since they have gibberish names, just keep going into folders and opening the icon.png to see what that webclip is for.
Once you find it, edit the info.plist. Near the bottom, replace "http://www.google.com" with "file:///var/mobile/wherever/the/file/is/file.pdf".
You now have a google webclip with a google icon that will open your pdf when clicked.
Now, this is the cool part...
Click on the google webclip and let it load your pdf. Now create a webclip. The webclip icon will be a preview of the pdf file and you can name it whatever you want. Then you can delete the google webclip.
Open the webclip and in the address bar you will see "file:///var/mobile/wherever/the/file/is/file.pdf" If you want another webclip for a different file, just change the address to point to that file, click "go", and create another webclip after that file is loaded.
Again, you can only view the files, not edit them, so I hope it helps.