Thank you!
In case some one else needs:
in beginning of theme I did add code to set business_area variable
{% for field in custom_fields %}
{% if field.label == 'Business Area' %}
{%- assign business_area = field.text | strip_newlines | rstrip -%}
{% break %}
{% endif %}
{% endfor %}
...
to remove unnecessary spaces and breaks I had to add
| strip_newlines | rstrip
and later in template I use it for switching logo
...
{% if business_area == 'Amberoot' %}
<td style="text-align: right"><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAKUAAAAkCAYAAADox/qnAAAACXBIWXMA=" style="max-height: 150px; max-width: 300px"></td>
{% else %}
{% if business.logo != null %}<td style="text-align: right"><img src="{{ business.logo }}" style="max-height: 150px; max-width: 300px"></td>{% endif %}
{% endif %}
...
image converted to base64 using http://websemantics.co.uk/online_tools/image_to_data_uri_convertor/result/
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAKUAAAAkCAYAAADox/qnAAAACXBIWXMA=
hope someone finds it helpful
Link to liquid documentation:
https://shopify.github.io/liquid/basics/introduction/
Also list of all variable in default themes:
Document Specific: title description fields (array of document fields like Invoice date, Invoice number, Order number etc.) field.label field.text
Recipient Details: recipient.name recipient.code recipient.address recipient.identifier
Business Details: business.name business.address business.identifier business.logo Table Details table table.columns column.align column.label table.rows row.cells cell.text table.totals total.emphasis total.label total.text
Custom Fields custom_fields field.label field.text
Emphasis (Used for labels like "PAID IN FULL") emphasis.positive emphasis.negative emphasis.text