Does your employer has given out detailed roles and responsibilities on a letterhead. If yes, you are good to go.
If no, then getting this on a letterhead that giving out such letters is not a company policy or some kind of country guidelines prevent them from giving such letters
I am not sure how big names could influence the process. I myself worked for big name pharmaceutical companies and I am currently working for one of the largest healthcare firms. It's been 3.5 months since my application and I haven't got the rejection (Hands crossed!!). Just an FYI, I haven't received standard reference letter, however, I had the email conversation with the previous HR (of big pharma company) and she said EU Guidelines prevent them from giving out such letters. I attached that email conversation with my payslips, contract letter, resignation letter and awards I received via email. I currently work for another big pharma firm which has the similar policy (though no country guidelines), they have given this on letterhead that they can't issue detailed roles and responsibilities and again here I have attached my payslips, contract letter and email communication with HR.
Also, I know my friend who has got the PR and he worked for the known company Facebook in India, and he didn't get roles and responsibilities on the letterhead. He worked with them for 4 years and he got the PR with contract letter, paystubs and HR communication that Facebook India does not issue such letters. Now he is working for KPMG in Canada....I hope this helps.
People usually get letters from fellow colleagues, but in my opinion HR communication stating such letters are not issued on your email is more authentic then fellow colleagues notarized declarations! In case you attach everything like paystubs, contract letter, Income tax documents, I don't think they will say that they can't consider this as your work ex. This works out for known firms (I wouldn't recommend this if your is local firm or small company)