If you’re going to get involved in digital reading you need to understand something about the technology.

PDF – this file will give an accurate reflection of what it captures. So it’s the best quality reproduction you will get. On the downside, the file sizes are HUGE – you’re looking at 12MB instead of 400KB file.  

If you are reading via dedicated ebook standards,  Kindle (mobi/azw)or epub files (and their variants) you will get smaller file sizes but often more corrupt scans.  Unless someone has re-edited the entire content you’ll get repeated errors such as u for n  c for e and the like. And often the spacing is random.  Frequently the text is absolutely unreadable.  Beware of course that some of the digital downloads will have been ‘edited’ for the length of the ‘free sample’ and beyond that…  there are some unscrupulous businesses out there remember.
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There are also some good guys in the value for money stakes. I should give a shout out for Delphi classics ebooks. These offer real value for money. Sadly, not all unco authors are covered.  

Read online. If you have the connection and the portable device, reading online means that you can access PDF (which is as close a file format to the original as you will get) which will give you the best quality available (which isn’t always acceptable quality!)

Free downloads. Again, if you have infinite space and a superfast connection, no problem. You can download huge files and read at your leisure.
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BUT be aware that when you’re getting ‘free’ you may be getting something that is actually worthless. Not always, but often.  The scan will at best only be as good as the original its scanned from and OCR struggles massively with 19th century (and earlier) fonts.  If you have a public domain work that’s been reprinted many times in the 20th century someone will be able to scan accurately… but these tend not to be the books we’re looking for.  ‘Rare’ books are less likely to be good quality for free. Get used to the idea.  You can waste a lot of good reading time trying to track down an acceptable digital download.

For free that’s not too much hassle, but you can pay good money for something that’s just as poor quality. You need to do your research on the purported publisher before you commit time and money to their download. 

If you have an academic affiliation the prospect is a lot brighter.  Which seems unfair, but at the moment there are still many gatekeepers between you and many unco writers.