I put out a little challenge for readers to solve to gain access to the PDF/EPUB links of Petit Bourgeois Volume 6, as an April Fool’s Joke. Unfortunately, my personality is quite alike that of Kobato, and I do occasionally enjoy testing the wisdom of others.
You can still find the challenge here, if you’re interested. And of course, if you want to try it, please do not read on.
The Solution

This is the key to the first hint. “Source” refers to the page source, so you need to click the settings button on your browser and click on “View Source” or use Ctrl + U. (On mobile,you need to add “view-source:” in front of the page url.) You’ll see something like this:

Looks like quite a mess, but if you scroll down, you’ll see some text that looks like what Kobato and Osanai are saying on screen, and below that…

The Osanai April Sweets Selection! It reads:
1 Summer-Exclusive Tropical Parfait
1 Paris Macaron
2 Spring-Exclusive Strawberry Tarts
3 Winter-Exclusive Chocolate Bonbons
1 Autumn-Exclusive Kuri-Kinton
These are, of course, the dessert names from each volume of the series. And where can you find all of these desserts? In the contents page, which there is a link to at the top and bottom of the Osanai April Challenge page.
In the contents page, hover over the cover images of each book to see their text (in mobile, you can click on them for the same effect).

Looking through all the pictures, we have:
Spring-Exclusive Strawberry Tarts: Radically Tasteful
Summer-Exclusive Tropical Parfait: Tangy, Succulent, Bittersweet
Autumn-Exclusive Kuri Kinton: Underneath, A Firey Experience
Paris Macaron: International Flavors Galore!
Winter-Exclusive Chocolate Bonbons: Miscellany of Nostalgic Palates
Pretty good descriptions of each volume that sound like they could be used for desserts, if I may say so myself (The description for Volume 5 is a little lacking because I have not finished reading it yet).
Anyway, what you do is take the corresponding number of letters from each description and put them in order according to the Osanai April Sweet Selection. We get:
Tangy, Succulent, Bittersweet
International Flavors Galore!
Radically Tasteful
Miscellany of Nostalgic Palates
Underneath, A Firey Experience
Tiramisu! Just pop that behind the pigcow-translations address and you get https://pigcow-translations.com/tiramisu/. If that doesn’t work a search bar will appear and by searching Tiramisu there, the page containing Volume 6’s PDF and EPUB links will show up.
Incidentally, tiramisu is the first dessert shown on the first episode of the anime.

All planned.
Anyway, hope you continue enjoying the story, and see you in the next chapter of the Berliner Bollen story in Volume 5.
Damn, I opened inspect source, ctrl+f for PDF, couldnt find it and gave up.