The Internet continues to weigh on HP Inc.'s (HPQ - Get Report) highly profitable printing unit -- and not just in the ways that the company blames for its latest results.

HP fell over 17% on Thursday after it missed January quarter (fiscal first quarter) revenue estimates (EPS was in line with expectations) and issued somewhat soft April quarter EPS guidance. The company did reiterate fiscal 2019 (ends in October) EPS guidance of $2.12 to $2.22 -- stock buybacks, cost cuts an lower memory prices are helping out -- but also said it now expects printing supplies revenue to be down about 3% in fiscal 2019, after previously forecasting it would be "flat to slightly up."

Though many consumers are more familiar with its PC business, HP's Printing segment produces more than twice as much pre-tax income for the company. And the lion's share of this income is believed to come from the sale of supplies such as ink and toner, rather than printer sales.

After having risen 7% annually in the October quarter, HP's supplies revenue fell 3% in the January quarter to $3.27 billion, missing a $3.42 billion consensus. On the earnings call, CEO Dion Weisler noted supplies sales were particularly weak in the EMEA region, where they fell 9%.

Weisler partly blamed HP's supplies shortfall on the fact that more businesses are buying printing supplies online, where its share (though market-leading) is lower than it is within offline distribution channels. He also claimed macro pressures have led customers to become more price-sensitive, and that this is impacting both HP's supplies share and pricing.

In addition, Weisler and CFO Steve Fieler said HP recently learned -- thanks to better market share data -- that it has too much supplies inventory in its channel. As a result, the company plans channel inventory cuts that are expected to hurt its fiscal 2019 supplies revenue by about $100 million, or 1%.

Though it's certainly possible that a shift towards online sales is hurting HP's supplies revenue, it's hard to believe that this shift would cause a massive change in HP's quarterly supplies growth rate more than 20 years after Amazon.com  (AMZN - Get Report) and eBay (EBAY - Get Report)  were founded. Likewise, though macro pressures could be impacting supplies pricing a bit, it's worth noting that many other big enterprise IT firms, including Microsoft (MSFT - Get Report) , Cisco Systems (CSCO - Get Report)  and HP Enterprise (HPE - Get Report) , haven't reported seeing large macro headwinds to date.

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There's quite likely another culprit at work here, one that had much to do with the revenue declines that the printing business saw each fiscal year from 2011 to 2016: Online and mobile document and photo-viewing are hurting printing demand across a number of end-markets.

Whether one is talking about consumers uploading photos to Facebook, iCloud and Google Photos instead of getting prints developed, office workers viewing PDFs and PowerPoint slides on their PCs and iPads rather than printing them out or newspapers seeing their revenue mix continuing to shift from print sales to digital subscriptions, the headwinds created for the printing hardware and supplies industry by the Internet and mobile devices remain enormous. And there isn't any reason to think that they'll be going away in the coming years.

HP, to its credit, has been well-aware of these long-term pressures, and has taken a number of steps to lessen their impact. These include growing its printing services revenue, both via managed printing services and its Instant Ink subscription business, as well as entering the 3D printing market with innovative offerings and growing its portfolio graphics printers and large-format A3 printers.

However, given the secular growth pressures that the printing industry faces, it's still not easy for a player of HP's size to maintain positive long-term growth for its printing business. At the very least, maintaining positive growth in the face of such pressures requires avoiding the kind of market share and macro issues that HP is blaming for its recent supplies weakness.