>>Naomi, just to make sure I understood correctly: you're looking for a way to prettify an XML document?
>
>I want to be able to see it not as huge big chunk of data, but normally with the tree structure (and have an ability to change something when looking at it this way).
>
>Also, files may be huge (~900KB or more)

Naomi, you can transform the XML using a stylesheet transformation (that's what Altova does, probably). The size may pose a problem for a MSXML2 parser, but the transformation can be used with a more capable processor (like .NET's).

In VFP (but, remember, the transformation can be used elsewhere):

LOCAL XML AS MSXML2.DOMDocument60
LOCAL XSLT AS MSXML2.DOMDocument60
LOCAL XMLPrettifier AS String
LOCAL Prettified AS String

m.XML = CREATEOBJECT("MSXML2.DOMDocument.6.0")
m.XSLT = CREATEOBJECT("MSXML2.DOMDocument.6.0")

m.XML.LoadXML("<root><child><grandchild>Vera</grandchild><grandchild>Chuck</grandchild><grandchild>Dave</grandchild></child></root>")

TEXT TO m.XMLPrettifier NOSHOW FLAGS 1
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns="http://www.w3.org/1999/xhtml" version="1.0">

    <xsl:output encoding="utf-8" method="text" indent="no"/>

    <xsl:variable name="spaces" select="'  '" />
    <xsl:variable name="NL" select="'&#13;&#10;'" />

    <xsl:template match="/">
        <xsl:apply-templates select="node()">
            <xsl:with-param name="indent" select="''"/>
        </xsl:apply-templates>
    </xsl:template>

    <xsl:template match="node()">
        <xsl:param name="indent"/>

        <xsl:value-of select="$indent"/>

        <xsl:text>&lt;</xsl:text><xsl:value-of select="name(.)"/><xsl:apply-templates select="@*"/>

            <xsl:if test="not(node())"><xsl:text> /</xsl:text></xsl:if>
        <xsl:text>&gt;</xsl:text>

        <xsl:if test="node()">

            <xsl:if test="node()[node()]">
              <xsl:value-of select="$NL" />
            </xsl:if>

            <xsl:apply-templates>
                <xsl:with-param name="indent" select="concat($indent, $spaces)"/>
            </xsl:apply-templates>


            <xsl:if test="node()[node()]">
                <xsl:value-of select="$indent"/>
            </xsl:if>

            <xsl:text>&lt;/</xsl:text><xsl:value-of select="name(.)"/><xsl:text>&gt;</xsl:text>
        </xsl:if>

        <xsl:value-of select="$NL" />

    </xsl:template>

    <xsl:template match="@*">
        <xsl:text> </xsl:text>
        <xsl:value-of select="name(.)"/>
        <xsl:text>=</xsl:text>
        <xsl:value-of select="concat('&quot;', ., '&quot;')"/>
    </xsl:template>

    <xsl:template match="text()">
        <xsl:value-of select="normalize-space(.)"/>
    </xsl:template>

    <xsl:template match="comment()">
        <xsl:text>&lt;--</xsl:text><xsl:value-of select="."/><xsl:text>--&gt;</xsl:text>
    </xsl:template>

    <xsl:template match="processing-instruction()">
        <xsl:text>&lt;?</xsl:text><xsl:value-of select="name(.)"/><xsl:text> </xsl:text><xsl:value-of select="."/><xsl:text>?&gt;</xsl:text>
        <xsl:value-of select="$NL" />
    </xsl:template>
</xsl:stylesheet>
ENDTEXT

m.XSLT.LoadXML(m.XMLPrettifier)

m.Prettified = m.XML.Transformnode(m.XSLT)

? m.Prettified

Credits: transformation stylesheet from https://gist.github.com/Jalalhejazi/5550511, with small changes.