The Indian pharmaceutical industry has grown in recent years to become a major manufacturer of health care products to the world. India produced about 8% of the global pharmaceutical supply in 2011 by value, including over 60,000 generic brands of medicines.[174] The industry grew from $6 billion in 2005 to $36.7 billion in 2016, a compound annual growth rate (CAGR) of 17.46%. It is expected to grow at a CAGR of 15.92% to reach $55 billion in 2020. India is expected to become the sixth-largest pharmaceutical market in the world by 2020.[175] It is one of the fastest-growing industrial sub-sectors and a significant contributor of India's export earnings. The state of Gujarat has become a hub for the manufacture and export of pharmaceuticals and active pharmaceutical ingredients (APIs).[176]
Or, there is another theory for your primary salary – generate enough to have a little excess cash flow, but do it at a place that you can work stress free and have time to dabble in other projects.  A good friend of mine has this setup – he works 10-5 and makes $50,000 a year.  This allows him to easily cover all of his expenses, but the shorter hours and flexibility in his job allows him to pursue his secondary income generating ideas!
Real Estate Income – Since we moved up to Newport Beach, I started renting out my condo in San Diego. My monthly cash on cash return is $300(I charge $1,900 for rent and my total payments including mortgage, HOA, property tax and insurance are $1,600) but I also get back around $350 every month in principal and about a $150 tax savings per month. But even this income is inconsistent, since sometimes expenses will pop up like last month when I had to buy a new A/C for $3,000!
One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.

Infectious diseases continue to blight the lives of the poor across the world. An estimated 40 million people are living with HIV/AIDS, with 3 million deaths in 2004. Every year there are 350–500 million cases of malaria, with 1 million fatalities: Africa accounts for 90 percent of malarial deaths and African children account for over 80 percent of malaria victims worldwide.Source 9
The U.S. Internal Revenue Service categorizes income into three broad types, active income, passive income, and portfolio income.[1] It defines passive income as only coming from two sources: rental activity or "trade or business activities in which you do not materially participate."[2][3] Other financial and government institutions also recognize it as an income obtained as a result of capital growth or in relation to negative gearing. Passive income is usually taxable.
Investor Income: This is the apex of income that one can achieve, you just have to live off your investments which you do not have to work much for once you have researched it thoroughly. Investors generate money by investing in other businesses, dividend paying equity shares, rental possessions, and other investments that need lesser amount of work. This work provides unlimited earning potential as they build their wealth as much faster rate than any other income categories.
I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.
  (3) The ratio between the deductions and the gross income is obtained through a U.S. personal income tax calculator [t] and reflect the 2005 income tax rates in the U.S. The federal income tax, social security tax, medicare tax, and state income tax are included. The deductions are taken as the average of the state of New York and the state of Arizona [t], and are representative of married without child status. The deductions on the average wage are confirmed through the average gross and net income of a household in the U.S.
Stocks (shares) paying dividends are typically a reliable source of income but they have higher risk of capital losses than cash and bonds. So, it’s wise not to chase yield indiscriminately.  The 10 highest yielding stocks on the ASX 200 (the 200 largest companies in Australia) are shown in the table below.  But their share price performance on average over the last year has been underwhelming, as shown in the far right column: