Manga is awesome on the PRS-505. Western comics work too, but not as well because their pages are bigger, so you might have to squint a bit. Manga tends to come in three formats:

• .cbz - this is a bunch of individual pictures given numerically-ascending filenames like page001.jpg, page0002.jpg and so on, all zipped up together in a .zip file, and then someone's right-clicked on it and changed .zip to .cbz. That's it. If you change the file extension back to .zip then you can open it in your file manager.
• .cbr - this is the same as .cbz but it started as a .rar instead of a .zip. The difference between .rar and .zip is that you need to install extra software to unzip .rar files. I'm sure that some nerd will be mad at me about that sentence and send me a long email about compression algorithms but whatever.
• .pdf - this is a pain in the arse, but we can still work with it (see later section).

If you put a .cbz file into Calibre, right-click and select Convert, then Calibre will automatically:

• Resize each image to exactly the right size for your screen;
• Recolour each image so that it uses 8 shades of grey just like your screen;
• Sharpen each image to make text more legible and offset any weird fuzziness brought in by the resizing process;
• Create a .epub file out of these processed images.

The .cbz to .epub conversion system is the magic that makes manga great on the PRS-505. So much so that it can sometimes be worth doing a two-step conversion process to make a .cbz out of some other sort of file, and then feeding that .cbz through the .cbz to .epub converter. It's that awesome.

Clicking on the “Comic Input” button on the conversion screen will let you see some options; in my experience, most comics look better with the Sharpen feature turned on, a few look better with it turned off, experiment for the best results. This feature can take a LOT of RAM if you're working with huge .cbz files so close any other programs except Calibre (especially your web browser) before you run it. When working with manga, make sure you select the “Right to left” checkbox appropriately, for when the manga throws in the occasional landscape page (so you don't end up erroneously looking at the left side of the page first).

(note: although images inside the books will be resized and greyscaled properly, cover images will still be huge and full-colour, so don't forget to hit that Resize Cover button too)

Even after having all its images resized to fit your screen, manga takes up ENORMOUS amounts of space, around 30 or 40 megabytes per volume. You can fit hundreds (thousands if you go without covers) of straight-up text books in the space taken up by a single manga volume. Just something to keep in mind.

PDFs are supposed to be a way to make sure that printed documents look the same regardless of the computer that's used to print them. PDFs are for things that you're actually going to print out on real physical paper. For anything else, they kinda suck, because they're basically made out of a photo of each page, and photos take up way more memory than text (and you usually can't do things like, say, copy-paste any text in the photo into a text editor, or resize the text without having to scroll horizontally, because it's not actually text, it's a picture of text).

If you really can't get a comic in any other format than .pdf, then here's what to do. Use the right-click→Convert Book option to convert the .pdf to a .zip file! That'll give you a .zip full of pictures with numerically-ascending filenames like page001.jpg, page002.jpg. Sound familiar? That's a couple clicks away from being a .cbz!

Go into your file manager and make a folder somewhere, doesn't matter what you call it, doesn't matter where you put it so long as you can get to it easily, this is just a temporary working folder. I'm gonna assume you call it “pdfshenanigans” for the sake of brevity.

Right-click on the book in Calibre, select “Save specific format to disk” and save the .zip to the “pdfshenanigans” folder. Then, open the “pdfshenanigans” folder in your file manager, find the .zip file that Calibre put in there (it might be inside an author folder or a title folder or something), right-click, rename, and change the “.zip” to “.cbz” - if your operating system tries to warn you not to do this, ignore it and do it anyway. Now, you can put that .cbz into Calibre and convert it to .epub as above, with all the image-manipulation magic!

Sometimes, instead of getting a big file called, say, “My Manga Vol. 01”, you might find yourself with a bunch of little files called “My Manga Vol. 01 Chapter 01,” “My Manga Vol. 01 Chapter 02” and so on. If you want, you can merge these chapters together into a single volume using the “EpubMerge” plugin by Jim Miller (install it from Preferences → Advanced → Plugins). Merge chapters into volumes - but don't be tempted to merge all the volumes into one big doorstop of a book! (see note elsewhere in the guide on SD card corruption)

Bonus material: fanfic-to-epub, multiple libraries for multiple jobs