How to delete blank pages in Rmarkdown(pagedown)

Like people in the comments said, the only way you can solve this is by using your own style. It will be nice if they provide a built-in option to do that in the future.

/* page size */
@page {
  size: 6in 9in;
}

/* store some string variables */
.shorttitle1 {
  string-set: h1-text content(text);
}

.shorttitle2 {
  string-set: h2-text content(text);
}

/* left page */
.running-h1-title {
  position: running(runningH1Title);
  width: var(--running-title-width);
  text-overflow: ellipsis;
  overflow: hidden;
}
.running-h1-title:before {
  content: string(h1-text);
}

@page :first {
  @top-left {
    content: none;
  }
  @top-right {
    content: none;
  }
  @bottom-right {
    content: none !important;
  }
  background-image: var(--front-cover);
  background-position: center;
  background-size: contain;
  background-repeat: no-repeat;
}

@page :left {
  @top-left {
    content: counter(page);
  }
}

@page :right {
  @top-right {
    content: counter(page);
  }
}

/* Front cover */
.front-cover {
  break-after: page;
}

/* page breaks; aka CSS fragmentation */
.level1 {
  break-before: page;
}
.section > h1, .section > h2, .section > h3, .section > h4, .section > h5, .section > h6 {
  break-after: avoid;
}
.footnotes {
  break-before: always;
  break-after: always;
}
.figure {
  break-inside: avoid;
}

/* do not break captions */
caption {
  break-inside: avoid;
  break-after: avoid;
}

and using it like that:

---
title: "A Multi-page HTML Document"
author: "Yihui Xie and Romain Lesur"
date: "`r Sys.Date()`"
output:
  pagedown::html_paged:
    toc: true
    toc_depth: 3
    # change to true for a self-contained document, but it'll be a litte slower for Pandoc to render
    self_contained: false
    css:
      - custom.css
---

# Exercises 1 {-}

# Exercises 2 {-}

# Exercises 3 {-}