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Many articles consider PPP as byte-oriented, and others mention it as bit-oriented.

+----------+----------+----------+----------+------------
       |   Flag   | Address  | Control  | Protocol | Information
       | 01111110 | 11111111 | 00000011 | 16 bits  |      *
       +----------+----------+----------+----------+------------
               ---+----------+----------+-----------------
                  |   FCS    |   Flag   | Inter-frame Fill
                  | 16 bits  | 01111110 | or next Address
               ---+----------+----------+-----------------

I think the frame fields all are in byte format, and it should be clearly byte-oriented.
Can anyone say why PPP is byte-oriented as well as bit-oriented?

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The header and everything else in a PPP frame is byte-aligned - and I don't see anything that is bit-oriented.

PPP doesn't provide byte alignment (there's no sync header), see RFC 1662 section 3:

This figure does not include bits inserted for synchronization (such as start and stop bits for asynchronous links), nor any bits or octets inserted for transparency.

So, byte alignment needs to be provided by the underlying physical layer, usually a serial interface.

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