HackMaster. If you don't play HackMaster* I can understand that this review might not be for you, but I'm not looking at this strictly from an in-game perspective....
1st off the details on this item. The Manual of Critical Hits (and fumbles & mishaps) is a 172 page, 7.18 MB file that you can download for free at Kenzer & Company. The file is not secured in any fashion. The page size is 5.83" x 8.28", which is good for a tablet or booklet printing.
I'm certain that the file could have been optimized a bit better and reduced in size somewhat. The table of contents on page 3 is superfluous and the bookmarks are quite half-assed. A minor personal annoyance is when a PDF author doesn't bother to fill in any of the document properties & metadata.
Now normally these kinds of things would grant the loss of a couple stars, but I'm looking at this PDF from a different vantage point probably because I know the guy who created it. This isn't favoritism, but I know that this free PDF download wasn't something Steve Lawrence created for the HackMaster community. This was a GM game aid he made for his own personal use and it is intended to be used on a tablet. A few extra pages were added to get the page count divisible by four if someone wanted to actually print this up as a physical booklet. This possible secondary use as a printed booklet is the reason why the file is as large as it is. Of course, if you wanted to print this out, the full-color booklet will take up a LOT of ink.
Basically this PDF is a personal-use item that was made available for others. Since it wasn't designed for sale, or even for distribution (originally), it gets a bit of a pass on these "flaws".
The main reason I wanted to use this PDF for review is because of the creator's original intention of using the PDF format to create a useful game aid. The overwhelming majority of gaming PDFs are basically just digital versions of a printed product. Hopefully the PDF version is bookmarked so you can get to the section you need quickly, but even that is often a crap-shoot. What I rarely see, and never to the extent of this "Manual", is the use of internal hyperlinks. This whole product revolves around this extremely useful PDF tool.
I don't think I have another gaming PDF with this level of functionality on a tablet, which is the reason I wanted to review this product. I've had plenty of PDF products that could use internal hot-linking that could really amp-up the usefulness of the medium. Personally I love that I can use my tablet more and more as a GM, and it really saves on me having to lug around so many books to a game.
*Even in you do not play HackMaster, if you wanted to add a little crunch to your OSR game, adding these Critical Hits and Fumbles/Mishaps from the current version of HackMaster is easily accomplished. There isn't too much you'd have to tweak to make it work and it could add a lot of spice to your game.