I have to purchase DC Power Supply 1. Topward 6303D, 2. Caltron PR 3303SE which one is best

Both are generic OEM designs. I would not expect to find detailed information on either.

This means one company designed it and sells it under multiple brand names. Sometimes there are minor variations as parts get substituted. I've seen both, but based on the picture, the Caltron design is more common to me.

They're both linear supplies, so they should be straightforward to troubleshoot and repair even without a schematic.

As with all supplies, my hard requirement is to have an output on/off control. I hate supplies that only have an AC Mains control.

Take  a look at the Rigol DP832A.

Similar price (in UK) to Topward 6303D and similar power output (2 x 30V @ 3A, 1 x 5V @ 3A).

DIY repairing of high end power supplies (Keysight, Rohde & Schwarz etc) is never simple because they don't publish schematics, but their stuff tends to keep working.

With cheaper generic equipment such details often leak out, so it might be easier to maintain a no-name type supply yourself.

The Rigol is in between, decent quality and a close to low end price - I have one myself and it's been reliable. It does come with proper documentation and Ethernet, USB and RS232 ports as standard.

www.rigol-uk.co.uk/.../

MK

Thank you @baldengineer for your guidance. In fact i have limited choices available. Fortunately, I have found schematics of Topward 6303D power supply. Here is the link https://imgur.com/a/GPrRB6D

But couldn't find same for Caltron PR3303SE. But i think the Caltorn has an enable output or Output ON/OFF control. Cannot confirm... 

Thank you. Sir, But it is not available in our country.

What country is it?

I live in Pakistan.

Indeed, looks like it's only available via import or dropshippers.

The currency has plummeted there (even more than usual) in the past few months, by 25% or more compared to a couple of months ago. And it's so painful even shipping from overseas without paying off all sorts of fees. Now would be a good time for the OP to ask any friend or family member traveling from another country to buy from another country as a favor.