If you want to import the PDF ebook downloaded from your computer into the reading application "ibook" that comes with iOS8, you can follow the following steps:

Step 1: Start iTunes and click Edit-Preferences to enter.

Step 2: Find and check the "Book" item, and then click the "ok" button.

Step 3: Click the "Books" category in the item list at the top left of the "Database" interface, and then drag the PDF ebook from the computer to the iTunes edit box.

Step 4: Click the "iPhone" button in the upper right corner of the interface when finished.

Step 5: Click the "Book" column in the toolbar above, then click the "Synchronize Books" item, and finally click the "Apply" button at the bottom right.

Step 6: Click the "Clear and Synchronize" button in the dialog box that appears. Then iTunes will help us with the rest of the import work.

Step 7: Then start iBook on iPhone, and you can see the PDF e-book you just imported.