A reproducible example, called a reprex will provide more to go on. I can't tell how the rendering is being generated, so why it opens in Preview could be due to a number of reasons.
Check first, however, if Preview is the default app for Word documents in the Finder
Thanks for your reply and for your advice about reproducible example.
I checked that Preview isn't the default app for Word documents in the Finder.
I also ran this simple .Rmd example, and the results is the same.
---
title: "test"
output: word_document
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
## R Markdown
This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.
When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:
```{r cars}
summary(cars)
```
## Including Plots
You can also embed plots, for example:
```{r pressure, echo=FALSE}
plot(pressure)
```
Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.
Any suggestion?
Thanks in advance
> sessionInfo()
R version 3.6.0 (2019-04-26)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS 10.15.1
Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
locale:
[1] es_ES.UTF-8/es_ES.UTF-8/es_ES.UTF-8/C/es_ES.UTF-8/es_ES.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] reprex_0.3.0
loaded via a namespace (and not attached):
[1] Rcpp_1.0.3 ps_1.3.0 digest_0.6.23 R6_2.4.1 evaluate_0.14
[6] rlang_0.4.2 fs_1.3.1 callr_3.3.2 whisker_0.4 rmarkdown_1.17
[11] tools_3.6.0 xfun_0.11 yaml_2.2.0 compiler_3.6.0 processx_3.4.1
[16] clipr_0.7.0 htmltools_0.4.0 knitr_1.26
So, the behavior isn't specific to Catalina (I'm on Mojave.)
This raises the question, though, why does rendering in Preview rather than the Viewer pane or open in browser present a problem? After all, reading docx in a browser, when it can be done at all, isn't default behavior.
I'm also getting a similar problem - except in my case it's suddenly trying to open in Keka (file decompressor app) instead of Word, which doesn't work at all. It worked fine opening generated .docx files in Word until I upgraded to Catalina. Word is set as my Finder default for .docx files.
It seems like it may be related to this issue? I did switch to Microsoft 365 before upgrading to Catalina.