5. 2.3 Legal: Intellectual Property - Audio/Video Downloading

Moreover, there are many apps in the Appstore by which you can download anything and they are not rejected at all like "TDownloader", "Download", "iDownloader" and so on.

The fact that some similarly equivalent apps have been authorized is not an argument for the review board.

Rejection is about 2.3 LEGAL: INTELLECTUAL PROPERTY - AUDIO/VIDEO DOWNLOADING

And you just speak of pdf in your post.

What's the reason ?

Read carefully:

5.2.3 Audio/Video Downloading: Apps should not facilitate illegal file sharing or include the ability to save, convert, or download media from third-party sources (e.g. Apple Music, YouTube, SoundCloud, Vimeo, etc.) without explicit authorization from those sources. Streaming of audio/video content may also violate Terms of Use, so be sure to check before your app accesses those services. Documentation must be provided upon request.

Did you get the explicit autorization ?

It seems it is the point.

>view PDF of book that is freely avaliable in internet

The fact that something is available for viewing at a website does not mean that you are entitled to scrape that website, extract the information and display it. The displaying of copyrighted information is usually an infringement of the copyright. You may be able to display a WKWebView of the entire website (deep linking). It is unclear at the present time whether deep linking is a violation of copyright law.