PST file very (too) large

The best way to reduce size of a file that is too large is by compacting it, thus reducing its size. This can be done in the following way: 

1.  Go to File, click Data File Management.
2. Click and select your Personal Folder, then click Settings.
3. Go to General > Compact Now.
4. Clik OK, then Close.

Delete any items you don't need and you don't wish to keep. Make sure to delete them from Deleted Files as well, since once deleted files first go to Deleted Files folder, and only after deleting from that location you will get additional space / smaller file.

In Navigation Pane, right-click on Deleted Items and then click Empty Folder.

You can archive your older items.

1. Go to File > Cleanup Tools > Mailbox Cleanup.
2. Click View Mailbox Size.
3. Select Find items older than X days, enter a number between 1 and 999 and then click Find.
4. You can also select Find items larger than X kilobytes, enter a number between 1 and 9999 and click Find.
5. To move items to an archive click AutoArchive. 

Document will be saved in your Documents/Outlook Files folder.

You can view the size of Deleted Files folder by clicking View Deleted Items Size. To delete them, click Empty. You can also view the size of Conflicts folder by clicking View Conflicts Size; to delete them, click Delete.