For Dropbox user:
a. Click on "Open in App"
b. File to save into your Dropbox
c. Click on top right corner "Export Button" > "Open In" > "Open In iBook"
d. Catalogue will be saved in iBook for offline viewing

For Non-Dropbox user:
a. Click on top right corner "Download" button
b. Click on Direct Download
c. File will be download in Safari
d. Tap on anywhere on screen and top menu will appear
e. Click on "Open in iBook" on top right
f. Catalogue will be saved in iBook for offline viewing